The present application claims priority to Korean Patent Application No. 10-2008-114111, filed Nov. 17, 2008, the subject matter of which is incorporated herein by reference in its entirety.
The present invention relates to nickel-manganese (Ni—Mn) binary compounds useful as an electrode material for electrochemical supercapacitors and a method of preparing the same.
Generally, secondary batteries are capable of being recharged, miniaturized in size, and maximized in capacity. Recently, as the demand for portable electronic apparatus (such as small size video cameras, cellular phones, personal notebook computers, PDAs, electric vehicles, automotive subsystems and the like) has increased, improved secondary batteries have been widely developed as power supplies. Representative examples of such secondary batteries include lithium (Li)-based secondary batteries and electrochemical supercapacitors. The lithium-based secondary batteries are rechargeable batteries and show excellent chargeability that can store a large amount of electric energy (high energy density) in a unit weight or unit volume. However, they cannot effectively cope with charging and discharging at high current densities (under high loads), and thus, cannot have high power since the positive electrode active material used therein is inherently a poorly conducting material. Furthermore, since the lithium-based secondary batteries always require a binder, which is an insulating material, the binder interferes with the conductivity of the electrode, and thus the performance of the battery further deteriorates at high current densities.
In contrast, electrochemical supercapacitors are devices in which the electrolyte is placed between two electrode systems. While electrochemical supercapacitors can store and deliver charge in a time scale of the order of several tens of seconds, their ability to deliver charge in short times is dictated by the kinetics of the surface redox (oxidation-reduction) reactions, charge-discharge processes in the electrical double layers and the combined resistivity of the matrix and electrolyte. The electrochemical supercapacitors can have hundreds of times higher energy density than conventional capacitors and thousands of times higher power density than conventional batteries. It should be noted that energy storage in the electrochemical supercapacitors can be both Faradaic or non-Faradaic.
In both the Faradaic and non-Faradaic electrochemical supercapacitors, the capacitance is highly dependent on the characteristics of the electrode and the electrode materials used therein. Ideally, the electrode materials should have a large surface area, low electrical resistance (specific resistance) and a fast response speed.
As such electrode materials for the electrochemical supercapacitor, transition metal oxides which include ruthenium oxide (RuO2), iridium oxide (IrO2), cobalt oxide (CoO), molybdenum oxide (MoO3), tungsten oxide (WO3), manganese oxide (MnO2), nickel oxide (NiO) and the like have been widely used in the art. Electrochemical supercapacitors that utilize ruthenium dioxide (RuO2) as an electrode material have been found to deliver high energy densities and power densities. However, although RuO2 shows the highest specific capacitance (720 F/g) as compared with the other electrode materials, its application is very restricted due to the tendency of the electrodes to undergo self-discharge, potential recovery resulting in a decrease in cell voltage (and loss of power) over time, and the use of the expensive ruthenium metal. Further, NiO, CoO and MnO2 are problematic in terms of their poorly reproducible specific capacitance properties. Therefore, there is still a need to develop new transition metal oxides to be used as electrode materials that have high capacity and are cost-effective.
It has been reported that among the currently available electrode materials, ruthenium oxides and hydroxides have relatively high specific capacitance (Kuo-Hsin Chang, et al., Chem. Mater. 19: 2112-2119, 2007; Il-Hwan Kim and Kwang-Bum Kim, Electrochem. Solid State Lett. 4: A62-64, 2001). Specific capacitance of nanocrystalline powders and thin films of ruthenium hydroxides (RuO2.xH2O) can achieve 600-650 F/g at a voltage sweep rate of 20 mV/s. However, since the total amount of energy stored in a supercapacitor is proportional to the amount of electrode materials used therein, the ruthenium dioxide supercapacitor requires a significant amount of expensive ruthenium metals, which is an obstacle to commercialization.
PCT International Publication No. WO 2003/088374 and U.S. Pat. Nos. 5,986,876, 6,181,546 and 7,084,002 disclose that nanocrystalline or nanoporous nickel (II) oxides and hydroxides can be successfully used as an electrode material at a moderate discharge current and sweep rate, although the increase in sweep rate to 50-100 mV/s is usually accompanied by a decrease in specific capacitance to 125-135 F/g. U.S. Pat. Nos. 6,339,528 and 6,616,875 disclose a method of fabricating a supercapacitor electrode by mixing manganese (IV) hydroxide powders with carbon black or graphite and coating the resulting mixture on a metal current collector, which showing high specific capacitance over 200 F/g at a sweep rate of 20 mV/s. However, manganese hydroxides (MnO2.xH2O) with high capacity values are problematic in terms of the high fade rate of specific capacitance. Further, in case of fabricating this into a thin film electrode, the electrochemical specific capacitance suddenly decreases depending on the thickness of the thin film, and thereby results in lowering the total specific capacitance of the supercapacitor using the electrode.
Taking into account the unique electrochemical behavior of the oxides and hydroxides of various transition metals, it would be reasonable to expect that the complex oxides and hydroxides of the various transition metals will also exhibit similar characteristics. For example, the specific capacitance of Co—Al layered double hydroxides/hydroxocarbonates and Co—Si double hydroxides can achieve 230-250 F/g at a sweep rate of 5-10 mV/s, although their working voltage window is relatively narrow (0.1-0.6 V vs Hg/HgO) (PCT International Publication No. WO 2006/032183).
The present inventors have therefore conducted research to develop a new electrode material having a high specific capacitance, a wide working voltage window and a relatively low fade rate so as to improve the performance of the electrochemical supercapacitor, and found that, when nickel and manganese are used in a binary compound form such as nickel-manganese coprecipitated hydroxides, hydroxocarbonates and oxides, the thus obtained nickel-manganese binary compounds do not cause the deterioration in electrical properties, such as specific surface area, specific resistance, response rate and the like, exhibit high specific capacitance per unit area, and show a low capacitance fade rate during the reversible cycling. Therefore, the nickel-manganese binary compounds according to the present invention can be effectively used as an electrode material for an electrochemical supercapacitor.
One of the objectives of the present invention is to provide an electrode material for an electrochemical supercapacitor by using a binary compound of nickel and manganese which shows a high charge-recharge rate (power density), excellent specific capacitance per unit area, a low capacitance fade rate and improved cycle life.
In order to achieve the above objective, one embodiment of the present invention relates to a nickel-manganese (Ni—Mn) binary compound useful as an electrode material for an electrochemical supercapacitor, which is one of nickel-manganese coprecipitated hydroxides having a spinel-like structure, nickel-manganese coprecipitated hydroxocarbonates having a calcite-like structure, and nickel-manganese oxides having an ilmenite-like structure.
Another embodiment of the present invention relates to a method of preparing the above nickel-manganese binary compound in a nanocrystalline particle form by chemical coprecipitation and freeze-drying.
The embodiments of the present invention will be described in detail with reference to the following drawings:
The present invention provides nickel-manganese (Ni—Mn) binary compounds useful as an electrode material for an electrochemical supercapacitor.
Unlike the conventional electrode material employing individual hydroxides or oxides of nickel and manganese, the electrode material for an electrochemical supercapacitor according to the present invention is characterized as employing nickel-manganese (Ni—Mn) binary compounds in the form of nickel-manganese coprecipitated hydroxides, hydroxocarbonates and oxides.
The nickel-manganese binary compound of the present invention is one of nickel-manganese coprecipitated hydroxides having a spinel-like structure, nickel-manganese coprecipitated hydroxocarbonates having a calcite-like structure, and nickel-manganese oxides having an ilmenite-like structure.
As one of the nickel-manganese binary compounds according to the present invention, the nickel-manganese coprecipitated hydroxides having a spinel-like structure can be prepared by the following steps:
1) inducing the coprecipitation of nickel and manganese while adding dropwise a sodium hydroxide (NaOH) aqueous solution or a potassium hydroxide (KOH) aqueous solution to a nickel-manganese acetate (CH3COOH) aqueous solution or a nickel-manganese nitrate (HNO3) aqueous solution, to thereby obtain nickel-manganese coprecipitated hydroxide particles; and
2) separating the nickel-manganese coprecipitated hydroxide particles obtained in step 1) from the reactant by filtering, washing and freeze-drying the same, to thereby obtain amorphous nickel-manganese coprecipitated hydroxide powders in a nanocrystalline particle form.
In step 1), the acetate aqueous solution or nitrate aqueous solution in which nickel and manganese are dissolved in a molar ratio of 1:1.8 to 1:3, or 1:2 is prepared. Here, if the molar ratio of nickel and manganese dissolved therein is lower than 1:1.8 or exceeds 1:3, there may be problems with the specific capacitance of the supercapacitor being lowered.
The coprecipitation of nickel and manganese is then carried out by carefully adding drop by drop the sodium hydroxide aqueous solution or potassium hydroxide aqueous solution to the nickel-manganese acetate aqueous solution or nitrate aqueous solution prepared above, thereby generating nickel-manganese coprecipitated hydroxide particles. The coprecipitation in step 1) may be carried out at a temperature of 20 to 90° C. for 2 to 10 hours. Here, the sodium hydroxide aqueous solution or potassium hydroxide aqueous solution may be added dropwise in an amount of 35 to 45 parts by weight based on 100 parts by weight of the nickel-manganese acetate aqueous solution or nitrate aqueous solution. Further, the pH of the reactant may be maintained within the range of 9.5 to 10.5 during the coprecipitation.
In some embodiments of the present invention, the nickel-manganese coprecipitated hydroxide particles are prepared by adding dropwise 400 ml of the 1 M sodium hydroxide aqueous solution to 1,000 ml of the 0.2 M nickel-manganese nitrate aqueous solution in which nickel and manganese nitrates are dissolved in a molar ratio of 1:2 at 90° C. for 10 minutes, and inducing the coprecipitation of nickel and manganese at 20° C. for 2 hours. Here, the pH of the reactant may be maintained at 10 during the coprecipitation.
In step 2), the reactant obtained after the coprecipitation is filtered to separate the nickel-manganese coprecipitated hydroxide particles, which is followed by washing with deionized water and freeze-drying, to thereby obtain amorphous nickel-manganese coprecipitated hydroxide powders in a nanocrystalline particle form. The freeze-drying is essential to prevent the nickel-manganese coprecipitated hydroxide particles from forming hard agglomerates, making it possible to obtain products in a nanocrystalline powder form.
In some embodiments of the present invention, the washed nickel-manganese coprecipitated hydroxide crystals are subjected to freeze-drying under a vacuum of about 5×10−2 mbar by using a freeze dryer, to thereby obtain amorphous or semiamorphous nickel-manganese coprecipitated hydroxide powders in a nanocrystalline particle form. The thus obtained nickel-manganese coprecipitated hydroxides have a tetragonal spinel-like structure.
Further, as another embodiment of the nickel-manganese binary compounds according to the present invention, the nickel-manganese coprecipitated hydroxocarbonates having a calcite-like structure can be prepared by the following steps:
1) inducing the coprecipitation of nickel and manganese while adding dropwise a mixture of a sodium hydroxide (NaOH) aqueous solution and a sodium carbonate (Na2CO3) aqueous solution to a nickel-manganese acetate aqueous solution or a nickel-manganese nitrate aqueous solution, to thereby obtain nickel-manganese coprecipitated hydroxocarbonate particles; and
2) separating the nickel-manganese coprecipitated hydroxide particles obtained in step 1) from the reactant by filtering, washing and freeze-drying the same, to thereby obtain amorphous or crystalline nickel-manganese coprecipitated hydroxocarbonate powders in a nanocrystalline particle form.
In step 1), the nickel-manganese acetate aqueous solution or nitrate aqueous solution is prepared according to the same method as described above for the preparation of the nickel-manganese coprecipitated hydroxides. To the nickel-manganese acetate aqueous solution or nitrate aqueous solution is carefully added an equimolar mixture of a sodium hydroxide aqueous solution and a sodium carbonate aqueous solution so as to induce the coprecipitation of nickel and manganese, thereby generating nickel-manganese coprecipitated hydroxocarbonate particles. The coprecipitation in step 1) may be carried out at a temperature of 20 to 90° C. for 6 to 12 hours. Here, the equimolar mixture of a sodium hydroxide aqueous solution and a sodium carbonate aqueous solution may be added dropwise in an amount of 55 to 65 parts by weight based on 100 parts by weight of the nickel-manganese acetate aqueous solution or nitrate aqueous solution. Further, the pH of the reactant may be maintained within the range of 9.8 to 10.2 during the coprecipitation.
It has been found that it is possible to generate relatively crystalline products having a calcite-like structure according to the above method of coprecipitating nickel and manganese, which is known in the art as a method for synthesizing layered double hydroxides.
In some embodiments of the present invention, to the 0.2 M nickel-manganese nitrate aqueous solution in which nickel and manganese nitrates are dissolved in a molar ratio of 1:2 is added dropwise the 1 M equimolar mixture of a sodium hydroxide aqueous solution and a sodium carbonate aqueous solution 10 minutes so as to induce the coprecipitation of nickel-manganese at 90° C. for 6 to 12 hours. Here, the pH of the reactant may be maintained at 10 during the coprecipitation.
In step 2), the nickel-manganese coprecipitated hydroxocarbonate particles generated in step 1) are filtered, washed and freeze-dried according to the same method as described above for the preparation of the nickel-manganese coprecipitated hydroxides, to thereby obtain nickel-manganese coprecipitated hydroxocarbonate powders in a nanocrystalline particle form. The thus obtained nickel-manganese coprecipitated hydroxocarbonates have a relatively crystalline calcite-like structure.
Furthermore, as a nickel-manganese binary compound according to the present invention, the nickel-manganese oxides having an ilmenite-like structure can be prepared by isothermally heat treating the nickel-manganese coprecipitated hydroxides having a spinel-like structure or nickel-manganese coprecipitated hydroxocarbonates having a calcite-like structure at a temperature of 300 to 400° C. for 1 to 2 hours. The isothermal heat treatment causes the progression of the amorphization of the nickel-manganese coprecipitated hydroxides and nickel-manganese coprecipitated hydroxocarbonates to crystallization, resulting in a conversion into a crystalline phase with a hexagonal structure similar to ilmenite (FeTiO3). The thus obtained nickel-manganese oxides have a hexagonal ilmenite-like structure.
The electrochemical properties of the nickel-manganese coprecipitated hydroxides, hydroxocarbonates and oxides obtained according to the present invention are analyzed as follows.
The nickel-manganese coprecipitated hydroxides having a spinel-like structure, nickel-manganese coprecipitated hydroxocarbonates having a calcite-like structure and nickel-manganese oxides having an ilmenite-like structure as described above are observed under a scanning electron microscope (SEM) and a transmission electron microscope (TEM). As a result, it is found that the nickel-manganese coprecipitated hydroxides and hydroxocarbonates consist of poorly agglomerated isotropic and plate-like crystals having an uniform particle size of 10 to 50 nm, preferably, 20 to 30 nm, respectively, and such a transformation of spinel and calcite to ilmenite does not result in significant morphological change in the above hydroxides and hydroxocarbonates that still maintain their unique nanocrystalline properties.
The reversible charge/discharge of the nickel-manganese binary compounds according to the present invention can take place in an alkaline electrolyte solution at a potential in the range of −1.1 to 0.9 V by using an Hg/HgO reference electrode.
These results demonstrate that the nickel-manganese binary compounds of the present invention show a high specific capacity of 260 C/g or more and a wide voltage window up to 2 V at a high current density (I=70 mA/cm2), while exhibiting a relatively low capacity fade rate.
As described above, the nickel-manganese binary compounds according to the present invention do not cause the deterioration of electrical properties such as specific surface area, specific resistance, response rate and the like, exhibit excellent specific capacity per unit area, show low capacity fade rate during the reversible cycling, and thereby have improved cycle life characteristics as compared with the conventional lithium secondary batteries. Therefore, the nickel-manganese binary compounds according to the present invention can be effectively used as an electrode material for an electrochemical supercapacitor.
While the present invention has been described and illustrated with respect to a number of embodiments of the invention, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad principles and teachings of the present invention, which is defined by the claims appended hereto.
Number | Date | Country | Kind |
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10-2008-0114111 | Nov 2008 | KR | national |