KR101008899B1 - Cathode for secondary battery having porous positive active material layer comprising metal oxide nanoparticles and the fabrication method thereof - Google Patents

Abstract

The present invention relates to a secondary battery positive electrode having a porous positive electrode active material layer including metal oxide nanoparticles having high output characteristics even when charging and discharging with a large current, and a method of manufacturing the same. Spinning the mixed solution under an electric field to form a composite fiber web in which the metal oxide precursor and the polymer are mixed; Thermally compressing or thermopressing the composite fiber web; It provides a method for manufacturing a positive electrode for a secondary battery, characterized in that to form a porous positive electrode active material layer containing metal oxide nanoparticles by removing the polymer from the composite fiber web by heat-treating the thermal compression or heat pressurized composite fiber web. .

Metal oxide, transition metal oxide, composite transition metal oxide, lithium composite metal oxide, secondary battery, positive electrode active material, nanofiber, web, electrospinning

Description

Cathode for secondary battery having a porous positive electrode active material layer containing metal oxide nanoparticles and a method for manufacturing the same

The present invention relates to a secondary battery positive electrode having a porous positive electrode active material layer containing metal oxide nanoparticles having high output characteristics even when charging and discharging at a large current, and a method of manufacturing the same.

Recently, the demand for secondary batteries continues to increase due to the power supply of portable transmitters, receivers, notebook computers, camcorders, digital cameras, and various home appliances. In addition, efforts are being made to use electric vehicles (EVs) or hybrid electric vehicles (HEVs) to alleviate environmental pollution caused by vehicle emissions and overcome the limitations of finite oil resources. In order to achieve this, an environmentally friendly battery having excellent cycle characteristics and high power density is required even if charging and discharging are repeated at a large current.

In general, the secondary battery includes a positive electrode having a positive electrode active material, a negative electrode having a negative electrode active material, a separator separating the positive electrode and the negative electrode, and an electrolyte that enables ion transfer between two electrodes.

In a lithium ion secondary battery using repeated insertion and desorption of lithium ions, the positive electrode active material is V ₂ O ₅ , CuV ₂ O ₆ , NaMnO ₂ , NaFeO ₂ , LiCoO ₂ , LiNiO ₂ , LiNi _1-y Co _y O ₂ , LiMn ₂ O ₄ , Li [Ni _1/2 Mn _1/2 ] O ₂ , LiFePO ₄ or Ligium Mg ²⁺ , Al ³⁺ , Ti ⁴⁺ , Zr ⁴⁺ , Nb ⁵⁺ , W ⁶⁺ Transition metal oxides such as LiFePO ₄ doped with ions of 1 at% or less are used, and in order to improve high output / high capacity characteristics, the cathode active materials listed above may be used in the form of a composite. As the negative electrode active material, metal materials such as Li, Si, Sn, metal oxides such as SnO ₂ , Fe ₂ O _3, and lithium intercalating carbon may be used. Mainly used.

In particular, in order to be used as a battery for EV and HEV, a high output characteristic that can be used for a long time with a single charge and can generate a large amount of current at one time is essential. In order to obtain high rate capability characteristics, a cathode active material and an anode active material having excellent electrical conductivity characteristics are required.

LiCoO ₂ , which is currently used as a cathode active material, has a low available energy density of about 140 mAh / g, and output is somewhat insufficient to produce high current density. In order to overcome this capacity limitation, Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ with a theoretical capacity value of about 278 mAh / g has been proposed [T. Ohzuke, Chemistry Letters, 642 (2001). In addition, research on materials of LiNi _1-x Co _x O ₂ , LiNi _1-x Ti _{x / 2} Mg _{x / 2} O _2, and the like has been actively performed. Further, in order to improve the electrical conductivity of the positive electrode active material to obtain high rate characteristics, studies are also being conducted to improve conductivity by substituting with other metal cations or mixing carbon, metal, or conductive metal oxide with the positive electrode active material. have.

In order to prepare such a cathode active material, solid phase reaction method, coprecipitation method, sol-gel method, hydrothermal synthesis method, combustion synthesis method, etc., which mix precursors containing each element together and undergo a high temperature heat treatment process, are actively attempted at present, It is commercially used.

However, the conventional cathode active material manufacturing methods listed above have to undergo a complicated addition process, have poor adhesiveness between the current collector and the cathode active material, and have a problem in that output characteristics are poor during charging and discharging with a large current. Accordingly, simpler coating techniques without the use of binders or additives are required to simultaneously achieve low cost and large area processes across various thicknesses, from thin films to thick films.

The present invention has been made to solve these conventional problems, the object of the present invention,

(1) It has an excellent electrical contact structure by controlling the size of the particles constituting the positive electrode active material layer as well as mutual networking between the particles, and has a high energy density by greatly increasing the specific surface area (reaction area) of the positive electrode active material layer. To provide a secondary battery positive electrode capable of fast charging and discharging under a large current and a method of manufacturing the same.

(2) improving the adhesion between the current collector and the positive electrode active material layer to provide a secondary battery positive electrode and a method of manufacturing the same having high mechanical, thermal and electrical stability;

(3) It is to provide a method for producing electrodes of various areas (particularly large areas) in a simple and inexpensive way over a wide thickness from a thin film to a thick film without adding a binder or an additive.

In order to achieve these purposes,

The present invention, the positive electrode current collector; Including a porous positive electrode active material layer comprising metal oxide nanoparticles formed by heat-treating and heat-compressing a metal oxide precursor-polymer composite fiber web accumulated by radiation in a state in which an electric field is applied to at least one surface of the positive electrode current collector It provides a secondary battery positive electrode, characterized in that made.

The present invention also provides a composite fiber web in which the metal oxide precursor (a precursor in the form of a metal salt) and the polymer are mixed by spinning a solution containing a metal oxide precursor and a polymer under an electric field on a positive electrode current collector; Thermally compressing or thermopressing the composite fiber web; It provides a method for manufacturing a positive electrode for a secondary battery, characterized in that to form a porous positive electrode active material layer containing metal oxide nanoparticles by removing the polymer from the composite fiber web by heat-treating the thermal compression or heat pressurized composite fiber web. .

The positive electrode active material layer produced by the present invention is well formed with mutual networking between fine nanoparticles, and the specific surface area is greatly increased, so that the charge and discharge efficiency of the battery is greatly increased, and in particular, the rate performance is possible to enable high-speed output. ) Is greatly improved, making it suitable for applications in high capacity / high power batteries.

In addition, the adhesiveness between the current collector and the positive electrode active material layer is greatly improved through thermal compression or thermal pressurization, so that a secondary battery having high mechanical, thermal, and electrical stability can be realized, and the thickness of the positive electrode active material layer can be easily adjusted by adjusting the spinning time. I can regulate it.

In addition, the type and ratio of the precursor including the metal salt can be easily adjusted, and as well as doping of a specific element to improve the electrical conductivity, it is possible to easily prepare a complex composite transition metal oxide of a multi-component system.

In addition, the secondary battery having the cathode active material layer manufactured by the present invention has a high initial discharge capacity and a charge capacity, and in particular, high output characteristics can be obtained even when charging and discharging with a large current. The positive electrode active material layer can be applied to medium and large secondary batteries mounted on power tools and electric vehicles.

The secondary battery positive electrode according to the present invention comprises a positive electrode current collector and a porous positive electrode active material layer.

The positive electrode current collector is (1) platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), rhodium (Rh), ruthenium (Ru), nickel (Ni), stainless steel , Aluminum (Al), molybdenum (Mo), chromium (Cr), copper (Cu), titanium (Ti) and tungsten (W) any one selected from the group consisting of, or (2) ITO (Sn doped In ₂ O ₃ ) or FTO (F doped SnO ₂ ) material, or (3) a metal material formed on a Si wafer. However, the present invention is not limited thereto.

The porous cathode active material layer includes metal oxide nanoparticles formed by thermally compressing and then thermally compressing a metal oxide precursor-polymer composite fiber web accumulated by radiation in a state where an electric field is applied to at least one surface of the cathode current collector. .

Herein, the porous cathode active material layer may have a web structure in which flat belt-shaped nanofibers including the metal oxide nanoparticles are entangled with each other, or the shape of the nanofibers disappears through thermal compression and heat treatment and mutually Networking may have a well formed structure.

The metal oxide nanoparticles may have a rod shape having an average width of 20 to 80 nm and an average length of 100 to 500 nm, and / or a round shape having an average size of 2 to 100 nm.

In the present invention, the metal oxide nanoparticles can be used as a positive electrode active material, oxide nanoparticles containing transition metals such as transition metal oxide nanoparticles, composite transition metal oxide nanoparticles, lithium composite transition metal oxide nanoparticles, and the like. An example is given. In this case, the term "composite transition metal" in the composite transition metal oxide nanoparticle includes not only a case where a transition metal and a transition metal are combined, but also a case where a transition metal and a non-transition metal are combined. In addition, the "lithium composite transition metal" in the lithium composite transition metal oxide nanoparticles refers to a case where lithium and a transition metal are combined.

Such metal oxide nanoparticles include (1) V ₂ O ₅ , CuV ₂ O ₆ , NaMnO ₂ , NaFeO ₂ , LiCoO ₂ , LiNiO ₂ , LiNi _1-y Co _y O ₂ (0 ≦ _y ≦ 0.85), LiMn ₂ O ₄ , Li [Ni _1/2 Mn _1/2 ] O ₂ and at least one selected from the group consisting of LiFePO ₄ or nanoparticles composed of two or more mixed phases, or (2) LiFePO ₄ , Li [Ni _{1 / 3} Co _1/3 Mn _1/3 ] O ₂ , Li [Ni _1/2 Mn _1/2 ] O ₂ , LiNi _1-y Co _y O ₂ (0 ≦ _y ≦ 0.85) or LiNi _1-x Ti _{x /} Mg ²⁺ , Al ³⁺ , Ti ⁴⁺ , Zr ⁴⁺ , Nb ⁵⁺ and W ^{6+ in} at least one lithium site selected from the group consisting of ₂ Mg _{x / 2} O ₂ (0 ≦ _x ≦ 0.85) At least one selected from the group consisting of ions may be nanoparticles doped at 1 at% or less, or (3) may be a mixture of (1) and (2). However, the present invention is not limited thereto, and a metal oxide having positive electrode active material properties (eg, Li / Li ⁺ of 2.4 V or more in the case of a lithium secondary battery) is not limited to a specific material.

On the other hand, the manufacturing method of the positive electrode for secondary batteries according to the present invention is largely (1) preparing a spinning solution, (2) manufacturing a metal oxide precursor-polymer composite fiber web through spinning, (3) thermal compression (heat pressurization And a step of preparing the porous cathode active material layer through heat treatment. Hereinafter, each step will be described in detail.

Preparation of Spinning Solutions

First, a metal oxide precursor that can be used as a positive electrode active material for spinning, a suitable polymer, and a solvent are mixed.

In the present invention, the metal oxide precursor means that it can be used as a positive electrode active material as a precursor that can have the form of a metal oxide through heat treatment. For example, the metal oxide precursor may include at least one or more precursors including transition metals.

As the metal oxide precursor, one or two kinds of precursors selected from precursors containing ions such as Li, Cu, Ni, Mn, Co, Fe, P, Mg, Zr, Na, Ti, Nb, Al, and W The above precursors can be mixed and used. That is, any one of the above-mentioned precursors containing ions or two or more selected, selected and mixed with the polymer solution and spun, and then subjected to a high temperature (for example, 400 ℃ or more) heat treatment V ₂ O ₅ Mg ²⁺ , Al in place of CuV ₂ O ₆ , NaMnO ₂ , NaFeO ₂ , LiCoO ₂ , LiNiO ₂ , LiNi _1-y Co _y O ₂ (0≤y≤0.85), LiMn ₂ O ₄ , LiFePO ₄ or Li LiFePO ₄ , Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ , Li [Ni doped with ³⁺ , Ti ⁴⁺ , Zr ⁴⁺ , Nb ⁵⁺ , W ⁶⁺ below 1 at% _1/2 Mn _1/2 ] O ₂ , LiNi _1-y Co _y O ₂ (0≤y≤0.85), LiNi _1-x Ti _{x / 2} Mg _{x / 2} O ₂ (0≤x≤0.8) Such as As long as the positive electrode active material can be formed, no specific precursor is restricted.

The polymer increases the viscosity of the solution to form a fibrous phase when spinning, and also serves to control the structure of the spun fiber by compatibility with the metal oxide precursor. Such polymers include polyurethane, polyetherurethane, polyurethane copolymers, cellulose acetate, cellulose acetate butylate, cellulose acetate propionate, polymethylmethacrylate (PMMA), polymethylacrylate (PMA), polyacrylic air Copolymer, polyvinylacetate (PVAc), polyvinylacetate copolymer, polyvinyl alcohol (PVA), polyperfuryl alcohol (PPFA), polystyrene, polystyrene copolymer, polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene Oxide copolymer, polypropylene oxide copolymer, polycarbonate (PC), polyvinyl chloride (PVC), polycaprolactone, polyvinylpyrrolidone (PVP), polyvinyl fluoride, polyvinylidene fluoride copolymer And at least one selected from the group consisting of polyamides. However, the present invention is not limited thereto, and the present invention is not particularly limited as long as it can be formed into an ultrafine fiber by an electrospinning method or the like.

As an example of the process for preparing the spinning solution, first, polyvinylacetate having excellent affinity with the vanadium oxide precursor is dissolved in dimethylformamide, acetone, detrahydrofuran, toluene, or a mixed solvent thereof, and is drawn by spinning under an electric field. A 5-20% by weight polymer solution is prepared that forms a suitable viscosity for formation. Polyvinylacetate uses a polymer having a weight average molecular weight of 500,000-1,500,000 g / mol. Next, the vanadium oxide precursor is added to the polymer solution in an amount of 1 to 60% by weight based on the polyvinylacetate polymer solution, acetic acid as a catalyst is added in an amount of 0.01 to 60% by weight based on the vanadium oxide precursor, and then room temperature. After reacting for 1 to 10 hours at, it is used as spinning solution. If necessary, add Cetyltrimethyl ammonium bromide (CTAB) and stir for several minutes. Here, CTAB serves as an additive to smooth the radiation by controlling the charge characteristics of the transition metal oxide precursor.

Preparation of Metal Oxide Precursor-Polymer Composite Fiber Web by Spinning

Next, on the positive electrode current collector, the spinning solution prepared above is spun under an electric field to form a composite fiber web in which the metal oxide precursor and the polymer are mixed.

In the embodiment of the present invention, the electrospinning method was used as a method for obtaining the composite fiber web, but the contents of the present invention are not limited thereto, and melt-blown, flash spinning, or electrostatic meltblown. It is also possible to use radiation by an electrostatic melt-blown method.

The electrospinning apparatus includes a spinning nozzle connected to a metering pump capable of quantitatively injecting spinning solution, a high voltage generator, and an electrode to form a spun composite web of fibers. A grounded metal plate (i.e. a positive electrode current collector) is used as the cathode, and a spinning nozzle with a pump with a controlled discharge rate per hour is used as the anode. For example, by applying a voltage of 10 to 30 kV and adjusting the solution discharge rate to 10 to 50 μl / min can be prepared an ultra-fine composite fiber web having a fiber thickness of 50 nm to 1000 nm. Electrospinning is performed until a superfine composite fiber web layer is formed on the current collector to a thickness of 0.5 to 100 μm.

Preparation of Porous Cathode Active Material Layer by Thermal Pressing and Heat Treatment

Next, the current collector in which the composite fiber web obtained by spinning under an electric field is laminated is thermocompressed. Thermal compression may be performed for 1 to 10 minutes at a temperature of at least the glass transition temperature of the polymer (eg, 60 ° C. or more when using polyvinylacetate as a polymer) by applying a pressure of 0.1 to 10 MPa. The pressure, temperature and time of the thermal compression may be appropriately selected in consideration of the type of polymer used and the glass transition temperature of the polymer. Alternatively, if it is possible to melt at or above the glass transition temperature of the polymer used, it is also possible to apply heat without a crimping step or pressurize using hot compressed air above the glass transition temperature of the polymer.

In this thermocompression or thermo-pressurization process, the flow between the metal oxide precursor and the polymer separated from the phase is suppressed, and is then formed into a network of fine nanoparticles through a heat treatment process. Depending on the compressive strength and the temperature applied to the current collector during compression, a network of nanofibers (hereinafter referred to as 'nanowire') may remain, and fine nanoparticles are uniformly distributed through the complete melting process of the polymer. It may have the form of a network.

This thermal compression or thermal pressurization process melts or partially melts the polymer in the composite fiber web, improves adhesion to the current collector, and increases the specific surface area and density per unit volume after heat treatment. It is possible to have an improved unique structure, thereby obtaining a positive electrode active material layer having a high C-rate density. The metal oxide nanoparticles not subjected to the thermal compression process fall off the current collector after the heat treatment, and it is impossible to evaluate the battery characteristics.

Subsequently, the thermocompression-bonded or pressurized composite fiber web is heat-treated to remove the polymer from the composite fiber web to form a porous cathode active material layer including metal oxide nanoparticles.

For example, when the polymer is decomposed and removed by heat treatment at 400 ° C. for 30 minutes in air after the thermocompression or thermopressing treatment, a metal oxide nanofiber network (hereinafter referred to as “web”) is obtained. The heat treatment temperature and time after thermal compression are determined in consideration of the crystallization and firing temperatures of the metal oxides used. Heat treatment may be performed in a temperature range of 400 ~ 800 ℃ depending on the type of metal oxide precursor. In addition, the cycle characteristics may be improved by obtaining a metal oxide nanofiber web having an amorphous structure through a low temperature heat treatment process.

Hereinafter, the present invention will be described in detail with reference to examples, but these examples are only presented to more clearly understand the present invention, and are not intended to limit the scope of the present invention. It will be determined within the scope of the technical spirit of the claims.

Example 1 Preparation of Vanadium Oxide Nanofiber Networks by Thermocompression and Post-Heat Treatment of Vanadium Oxide Precursor-Polyvinylacetate Composite Fiber Web Layer

1.5 g of polyvinylacetate (Mw: 500,000) was added to 15 ml of dimethylformamide (DMF) and dissolved in the polymer solution for about one day. Then, 3 g of vanadium acetylacetonate was added and stirred. At this time, 1 ml of acetic acid and 0.003 g of Cetyltrimethyl ammonium bromide (CTAB) were added in a small amount for smooth spinning.

The reacted precursor was transferred to a syringe (Syringe), mounted on an electrospinning apparatus, and a voltage was applied between the tip of the syringe and the current collector to form a vanadium oxide precursor-polyvinylacetate (PVAc) composite fiber web layer. Got it. Here, the voltage was 12 kV, the flow rate was 30 μl / min, the total discharge amount was 500-10,000 μl, and the distance between the tip and the current collector was about 10 cm. FIG. 1 shows a scanning electron micrograph of a vanadium oxide precursor-PVAc composite fiber web layer obtained on a current collector after electrospinning. It shows that the network structure of the nanowire of 100-300 nm size is well formed on the current collector.

When the vanadium oxide precursor-PVAc composite fiber web layer shown in FIG. 1 is directly heat-treated at a high temperature of 400 ° C. or higher, cracks are generated or adhesive properties with the current collector are poor, resulting in detachment from the substrate after the heat treatment. This occurred. This is because only a part of the composite fiber web layer is in line contact at the interface with the current collector substrate, thereby lowering the adhesive strength after heat treatment. In order to overcome this problem, the composite fiber web layer shown in FIG. 1 above the glass transition temperature of the used polymer was thermally compressed and subjected to heat treatment in part or in whole. FIG. 2 is a scanning electron micrograph obtained after thermally compressing a current collector on which a vanadium oxide precursor-polymer composite fiber web layer is laminated by electrospinning at a pressure of 0.1 MPa in a press heated at 60 ° C. for 60 seconds. At this time, it was found that the surface structure of the polymer changed depending on the temperature and pressure applied to the current collector. In addition, it was also possible to determine different thermocompression temperatures depending on the polymer used. It can be seen in FIG. 2 that the vanadium oxide precursor-PVAc composite fibers are connected to each other through the overall melting process of the polymer (PVAc) occurring during the thermal compression process. At this time, the adhesive strength is greatly increased after the heat treatment while the surface contact is made in contact with the current collector substrate rather than the line contact during the melting of the polymer.

3 shows a scanning electron micrograph of vanadium oxide (V ₂ O ₅ ) obtained after heat treatment at 400 ° C. for 30 minutes through thermal compression. In the present embodiment, the heat treatment at 400 ℃, it is also possible to heat treatment in the temperature range of 400 ~ 800 ℃ to control the particle size of vanadium oxide. According to FIG. 3, it can be seen that vanadium oxide in the form of a fine nanorod is well formed. In addition, the shape of the nanorod-type vanadium oxide in the transmission electron microscope (TEM) shown in Figure 4 can be confirmed well. Nanorods can also be seen as a form of nanoparticles, and for nanoparticles obtained after electrospinning, thermocompression, and heat treatment, no particular restrictions are placed on the shape of the particles. For example, round particles and / or rods elongated in one direction can be obtained. In the transmission electron micrograph of Figure 4 (a), it can be seen that the particles constituting the vanadium oxide anode active material is in the form of a nanorod having an average width of 20 ~ 80 nm, the average length of 100 ~ 500 nm range. 4 (b) shows that the vanadium oxide nanorod network heat treated at 400 ° C. has a structure in which partial crystalline and amorphous are mixed.

Example 2 Preparation of Lithium Cobalt Oxide Nanoparticle Networks by Thermocompression and Post-Heat Treatment of Lithium Cobalt Oxide Precursor-Polyvinylacetate Composite Fiber Web Layer

1.5 g of polyvinylacetate (Mw: 500,000) was added to 15 ml of dimethylformamide (DMF) and dissolved in the solution for about a day. Then, 0.39 g of lithium chloride and 1.61 g of cobalt acetate were added together and stirred. It was. At this time, 1 ml of acetic acid and 0.002-0.004 g of Cetyltrimethyl ammonium bromide (CTAB) were added in a small amount for smooth spinning.

The reacted precursor was transferred to a syringe, mounted on an electrospinning apparatus, and voltage was applied between the tip of the syringe and the current collector to obtain a lithium cobalt oxide precursor-polyvinylacetate composite fiber web layer. Here, the voltage was 12 kV, the flow rate was 30 μl / min, the total discharge amount was 500-10,000 μl, and the distance between the tip and the current collector was about 10 cm. 5 shows a scanning electron micrograph after the lithium cobalt oxide precursor-PVAc composite fiber is spun onto the current collector. It is shown that nanofibers of 300 to 700 nm are well formed in a network form on the current collector.

When the lithium cobalt oxide precursor-PVAc composite fiber is heat-treated at a high temperature of 450 ° C. or higher without a thermocompression bonding process, cracks are generated or adhesive properties with the current collector are poor, resulting in a problem of detachment from the substrate after the heat treatment. It was. In order to overcome this, heat treatment was performed after the thermal compression process above the glass transition temperature of the used polymer. FIG. 6 shows a scanning electron micrograph of a current collector on which a lithium cobalt oxide precursor-PVAc composite fiber web layer is laminated by electrospinning at 60 ° C. for 60 seconds at a pressure of 0.1 MPa. At this time, it was found that the surface structure of the polymer changed depending on the temperature and pressure applied to the current collector. In addition, it was also possible to determine different thermocompression temperatures depending on the polymer used. It can be seen in FIG. 6 that the lithium cobalt oxide precursor-PVAc composite fibers are connected to each other through the overall melting process of the polymer (PVAc) occurring during the thermal compression process.

FIG. 7 shows a scanning electron micrograph (x 30,000) of lithium cobalt oxide (LiCoO ₂ ) obtained after heat treatment at 450 ° C. for 30 minutes through thermal compression. In this embodiment, but the heat treatment at 450 ℃, in order to control the particle size of the lithium cobalt oxide it is also possible to heat treatment in the temperature range of 400 ~ 800 ℃. FIG. 8 is a magnified scanning electron micrograph (x 100,000) of FIG. 7 and shows that lithium cobalt oxide in the form of fine nanoparticles is well formed. In particular, when the thermal compression process is less than 30 seconds to less than 0.1 MPa at a temperature lower than 60 ℃, while maintaining a network of the nanowires observed in Figure 5, each of the nanowires are pressed and belted during the compression process Each nanobelt is composed of fine lithium cobalt oxide nanoparticles to form a network. Thus, a network of oxides in which transition metals and lithium are complex is basically a network of nanoparticles, but may also be a network of nanowires (this is an oxide network of transition metals only, a network of oxides in which transition metals and transition metals are complexed). The same applies to networks of oxides in which transition and non-transition metals are combined).

Example 3 Preparation of LiFePO ₄ Nanoparticle Networks by Thermocompression and Post-Heat Treatment of LiFePO ₄ Precursor-Polyvinylacetate Composite Fiber Web Layer

0.28 g of lithium chloride (Lithium Chloride, mw 42.39) and iron (III) chloride (Iron (III) in a polymer solution dissolved in 15 ml of polyvinylacetate (Mw: 500,000) in 15 ml of dimethylformamide (DMF) for about a day. ) chloride, mw 162.21) 1.04 g, and 1.68 g of Triphenylphosphine (mw 262.29) were added together and stirred. At this time, 0.002 g of Cetyltrimethyl ammonium bromide (CTAB) was added in a small amount for smooth spinning.

The reacted precursor was transferred to a syringe, mounted on an electrospinning apparatus, and a voltage was applied between the tip of the syringe and the current collector to obtain a LiFePO ₄ precursor-PVAc composite fiber web layer. Here, the voltage was 12 kV, the flow rate was 30 μl / min, the total discharge amount was 500-10,000 μl, and the distance between the tip and the current collector was about 10 cm. FIG. 9 shows a scanning electron micrograph after LiFePO ₄ precursor-PVAc composite fiber is spun onto a current collector. It shows that 200 ~ 600 nm composite fiber is well formed by forming the network structure of nanowires.

A thermocompression process was conducted to enhance the adhesive strength of the current collector and the composite fiber web layer. FIG. 10 is a scanning electron micrograph of a current collector in which a LiFePO ₄ precursor-PVAc composite fiber is spun and laminated to a current collector heated at 60 ° C. at a pressure of 0.1 MPa for 60 seconds. At this time, it was found that the surface structure of the polymer changed depending on the temperature and pressure applied to the current collector. In addition, it was also possible to determine different thermocompression temperatures depending on the polymer used. It can be seen in FIG. 10 that the LiFePO ₄ precursor-PVAc composite fibers are connected to each other through the overall melting process of the polymer (PVAc) occurring during the thermal compression process.

FIG. 11 shows a scanning electron micrograph (x 80,000) of LiFePO ₄ obtained after thermal treatment at 450 ° C. for 30 minutes. When thermally treated at 450 ° C., sufficient particle growth did not occur, resulting in a network shape composed of ultrafine nanoparticles of less than 15 nm. In comparison, FIG. 12 shows a scanning electron micrograph (x 50,000) of LiFePO ₄ heat-treated at 600 ° C. for 30 minutes through thermal compression. In FIG. 12, it can be seen that a network of LiFePO ₄ composed of 50 nm-sized nanoparticles is well formed. In this way, it is possible to produce a cathode active material network having a surface structure of a desired shape according to the change of thermal compression and heat treatment conditions.

[Experiment 1] Characterization of lithium secondary battery using vanadium oxide nanoparticle network as cathode active material

In order to confirm the cathode active material characteristics of the vanadium oxide (V ₂ O ₅ ) nanoparticle (nanorod) network formed on the current collector (stainless steel) substrate according to Example 1, a coin cell (CR2032-type coin cell) was as follows. The structure was prepared. In the cell configuration, an EC / DEC (1/1 volume%) solution in which 1 M of LiPF ₆ was dissolved was used as the electrolyte. In addition, as a cathode to be used as a reference electrode (relative electrode), metal lithium foil having a purity of 99.99% (Foote Mineral Co.) was used, and a vanadium oxide nanorod network thin layer obtained through Example 1 was used as a working electrode. Polypropylene film (Celgard Inc.) was used as a separator to prevent an electrical short between the cathode and the anode. The fabrication of such a cell was performed after creating an argon (Ar) atmosphere in a VAC glove box.

Here, the charge / discharge tester used was WonATech's WBCS3000 model, which examined the voltage change under constant current with the Multi Potentiostat System (MPS), which was able to measure 16 channels by adding 16 boards. The intensity of the current density used during charging and discharging was calculated based on 0.5 C-rate and 2 C rate by calculating the theoretical capacity of each material. In the case of the positive electrode active material of the thin vanadium oxide nanorod network, the cut off voltage was 2.4 V to 3.8 V.

FIG. 13 shows the change in discharge capacity with respect to the number of cycles when the porous vanadium oxide nanorod network heat-treated at 400 ° C. at a pressure of 0.1 MPa for 30 seconds at 60 ° C. after electrospinning at 0.5 ° C. Show measured results. A high capacity value of 400 mAh / g shows the characteristic observed up to 50 cycles. C-rate is defined as the current that flows when the capacity is released in one hour. Therefore, it can be said that the higher the C-rate, the higher the maximum current that can be instantaneously output. This is a very important factor in the power supply of electronic and mechanical devices that require high power instantaneously.

In order to confirm the high rate characteristics of the vanadium oxide nanorod network, charge and discharge characteristics were evaluated at 2C rate. 14 shows the results of charge and discharge characteristics measured at 2 C using the porous vanadium oxide nanorod network obtained through Example 1 as the positive electrode active material. As the C-rate increases, the discharge capacity decreases slightly. However, the property of maintaining a high capacity of 300 mAh / g or more without a significant decrease in capacity even after 50 cycles is noteworthy. Through this, it is possible to utilize not only high capacity but also as a positive electrode active material of a high output secondary battery.

In the above, the present invention has been described with reference to the illustrated examples, which are merely examples, and the present invention may be embodied in various modifications and other embodiments that are obvious to those skilled in the art. Understand that you can.

1 is a scanning electron micrograph of a vanadium oxide precursor-PVAc composite fiber web electrospun on a current collector.

FIG. 2 is a scanning electron micrograph after thermal compression of an electrospun vanadium oxide precursor-PVAc composite fiber web on a current collector.

3 is a scanning electron micrograph observed after heat-squeezing the vanadium oxide precursor-PVAc composite fiber web electrospun on a current collector and heat-treated at 400 ° C.

4 are transmission electron micrographs of vanadium oxide obtained by thermal compression and heat treatment after electrospinning on a current collector.

5 is a scanning electron micrograph of a lithium cobalt oxide precursor-PVAc composite fiber web electrospun on a current collector.

6 is a scanning electron micrograph after thermocompression bonding of a lithium cobalt oxide precursor-PVAc composite fiber web electrospun on a current collector.

FIG. 7 is a scanning electron micrograph of the lithium cobalt oxide precursor-PVAc composite fiber web electrospun on a current collector and heat-treated at 450 ° C. after thermal compression.

FIG. 8 is an enlarged scanning electron micrograph of FIG. 7.

9 is a scanning electron micrograph of a LiFePO ₄ precursor-PVAc composite fiber web electrospun on a current collector.

10 is a scanning electron micrograph after thermal compression of the electrospun LiFePO ₄ precursor-PVAc composite fiber web on the current collector.

FIG. 11 is a scanning electron microscope photograph of an electrospun LiFePO ₄ precursor-PVAc composite fiber web on a current collector, followed by heat treatment at 450 ° C.

12 is a scanning electron micrograph observed after heat-squeezing the LiFePO ₄ precursor-PVAc composite fiber web electrospun on a current collector and heat-treated at 600 ° C. FIG.

FIG. 13 is a charge / discharge cycle characteristic when the vanadium oxide nanorod network prepared through Example 1 of the present invention is used as a positive electrode active material, and shows the result measured at 0.5 C. FIG.

FIG. 14 shows the results of measurement at 2 C as a charge / discharge cycle characteristic when the vanadium oxide nanorod network prepared through Example 1 of the present invention is used as a cathode active material.

FIG. 15 shows discharge capacity characteristics according to the number of cycles when the vanadium oxide nanorod network prepared through Example 1 of the present invention is used as a cathode active material.

Claims (14)

A positive electrode current collector; Including a porous positive electrode active material layer comprising metal oxide nanoparticles formed by heat-treating and heat-compressing a metal oxide precursor-polymer composite fiber web accumulated by radiation in a state in which an electric field is applied to at least one surface of the positive electrode current collector A secondary battery positive electrode, characterized in that made. The cathode of claim 1, wherein the porous cathode active material layer has a web structure in which flattened belt-shaped nanofibers including the metal oxide nanoparticles are entangled with each other. The metal oxide nanoparticle of claim 1, wherein the metal oxide nanoparticle has at least a rod shape having an average width of 20 to 80 nm and an average length of 100 to 500 nm, and a round shape having an average size of 2 to 100 nm. A secondary battery positive electrode having a shape. The positive electrode for a secondary battery of claim 1, wherein the metal oxide nanoparticles are oxide nanoparticles containing a transition metal. The method of claim 4, wherein the metal oxide nanoparticles, (1) V ₂ O ₅ , CuV ₂ O ₆ , NaMnO ₂ , NaFeO ₂ , LiCoO ₂ , LiNiO ₂ , LiNi _1-y Co _y O ₂ (0 ≦ _y ≦ 0.85), LiMn ₂ O ₄ , Li [Ni _{1 / 2} Mn _1/2 ] O ₂ and LiFePO ₄ at least one selected from the group consisting of nanoparticles or two or more mixed phases, or (2) LiFePO ₄ , Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ , Li [Ni _1/2 Mn _1/2 ] O ₂ , LiNi _1-y Co _y O ₂ (0 ≦ y ≤ 0.85) or at least one lithium site selected from the group consisting of LiNi _1-x Ti _{x / 2} Mg _{x / 2} O ₂ (0≤x≤0.8), Mg ²⁺ , Al ³⁺ , Ti ⁴⁺ , At least one selected from the group consisting of Zr ⁴⁺ , Nb ⁵⁺ and W ⁶⁺ ions is nanoparticles doped at 1 at% or less, or (3) The positive electrode for secondary batteries, characterized in that (1) and (2) are mixed. The method of claim 1, wherein the positive electrode current collector is (1) platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), rhodium (Rh), ruthenium (Ru), nickel (Ni), stainless steel, aluminum (Al), molybdenum (Mo), chromium (Cr), copper (Cu), titanium (Ti) and tungsten (W) any one material selected from the group consisting of, or (2) Sn doped In ₂ O ₃ ), FTO (F doped SnO ₂ ), or (3) A metal secondary battery. Spinning a solution containing the metal oxide precursor and the polymer on the positive electrode current collector under an electric field to form a composite fiber web in which the metal oxide precursor and the polymer are mixed; Thermally compressing or thermopressing the composite fiber web; Heat-treating the thermally compressed or thermo-compressed composite fiber web to remove the polymer from the composite fiber web to form a porous cathode active material layer including metal oxide nanoparticles. The method of claim 7, wherein the metal oxide precursor comprises at least one precursor containing a transition metal. The method of claim 8, wherein the metal oxide precursor is at least one ion selected from the group consisting of Li, Cu, Ni, Mn, Co, Fe, P, Mg, Zr, Na, Ti, Nb, Al, and W ions. The manufacturing method of the positive electrode for secondary batteries containing 1 type (s) or 2 or more types of precursor containing these. The method of claim 7, wherein the polymer is polyurethane, polyetherurethane, polyurethane copolymer, cellulose acetate, cellulose acetate butylate, cellulose acetate propionate, polymethylmethacrylate (PMMA), polymethylacrylate (PMA) ), Polyacrylic copolymer, polyvinylacetate (PVAc), polyvinylacetate copolymer, polyvinyl alcohol (PVA), polyperfuryl alcohol (PPFA), polystyrene, polystyrene copolymer, polyethylene oxide (PEO), polypropylene oxide (PPO), polyethylene oxide copolymer, polypropylene oxide copolymer, polycarbonate (PC), polyvinyl chloride (PVC), polycaprolactone, polyvinylpyrrolidone (PVP), polyvinyl fluoride, polyvinyl It is at least one selected from the group consisting of a denfluolide copolymer and polyamide. Method of producing a secondary battery positive electrode according to. 8. The method of claim 7, wherein the radiation is radiation by electrospinning, melt-blown, flash spinning, or electrostatic melt-blown. The manufacturing method of the positive electrode for secondary batteries which makes it a. The method of claim 7, wherein the thermocompression step, the production of a positive electrode for a secondary battery, characterized in that to melt the polymer partially or entirely in the composite fiber web by applying a pressure of 0.1 ~ 10 MPa at a temperature above the glass transition temperature of the polymer. Way. The method of claim 7, wherein the thermo-pressurization is performed by heating at a temperature above the glass transition temperature of the polymer to induce melting of the polymer, or by pressurizing using compressed air having a temperature above the glass transition temperature of the polymer. A method of manufacturing a positive electrode for a secondary battery, which induces melting of a polymer. The method of claim 7, wherein the heat treatment is performed in a temperature range of 400 ° C. to 800 ° C. according to the type of the metal oxide precursor.

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