Patent Application
20240313207
This application claims the priority of Korean Patent Application No. 10-2023-0034016 filed on Mar. 15, 2023, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.
The present invention relates to a positive electrode active material for a sodium ion battery, which has excellent cycle stability and durability against humidity, a sodium ion battery containing the same, and a method for preparing a positive electrode active material for a sodium ion battery.
Sodium ion batteries have been explored as a potential alternative to lithium ion batteries that dominate the market at present. Typical positive electrode active materials for sodium ion batteries include polyanionic compounds and layered transition metal oxides.
The polyanionic compounds (e.g., phosphate, pyrophosphate, fluorophosphate, NASICON-structure, sulfate, silicate) are generally formed of tetrahedral anionic units interconnected with transition metal polyhedral units, and thus have high operational potential and cycle stability, but have unfavorable aspects such as low electronic conductivity and insufficient rate characteristics in the case of no surface or shape modification.
The layered transition metal oxides have relatively high electronic/ion conductivity, and thus are capable of fast charge/discharge, but have poor cycle stability due to multiple phase transitions caused upon charge/discharge and are also highly sensitive to humidity, making it difficult to manufacture electrodes or store the electrodes for the long term.
O3-NaCrO2 has a reasonable level of reversible capacity, nearly flat charge/discharge voltage, and high thermal stability, and thus is one of the most promising positive electrode active materials among the layered transition metal oxides, and O3-NaCrO2 also undergoes multiple phase transitions upon charge/discharge, and in this case, when Na+ is extracted up to Na0.4-0.5CrO2, phase transitions reversibly take place into O3/O′3, O′3, O′3/P′3, and P′3. In addition, further extraction of Na+ results in irreversible structural changes from a layered structure to a rock-salt structure along with concomitant chromium ion migration. Even when de-sodiation is limited to Na0.5CrO2 considering the above-described structural changes, capacity continues to decrease over repeated charge/discharge cycles, and generally, O3-NaCrO2 shows a capacity retention of less than 80% with respect to an initial capacity within 100 charge/discharge cycles.
The invention intends to provide a positive electrode active material for a sodium ion battery, which has excellent cycle characteristics and durability against humidity, and a sodium ion battery including the positive electrode active material.
The invention also intends to provide a method for preparing a positive electrode active material for a sodium ion battery, which may produce a positive electrode active material for a sodium ion battery, which has a core-shell structure and excellent cycle characteristics and durability against humidity through a one-pot process.
According to a first aspect of the invention, there is provided a positive electrode active material for a sodium ion battery, which includes core particles having a composition of Formula 1 below and a shell composed of chromium oxide formed on a surface of the core particles.
Na(Cr1-2xAl2x)O2 (x is 0.001 to 0.1) [Formula 1]
According to a second aspect of the invention, there is provided a sodium ion battery including a positive electrode, a negative electrode disposed at a predetermined distance from the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte charged between the positive electrode and the negative electrode, and the separator, wherein the positive electrode includes a positive electrode active material including core particles having a composition of Formula 1 above and a shell composed of chromium oxide formed on a surface of the core particles.
According to a third aspect of the invention, there is provided a method for preparing a positive electrode active material for a sodium ion battery, which includes dissolving a sodium (Na) precursor, a chromium (Cr) precursor, and an aluminum (Al) precursor in a solvent to prepare a solution, adding a chelating agent to the solution and stirring the mixture, drying the solution to obtain a compound containing sodium, chromium, and aluminum, heating the compound at a temperature of 400 to 600° C. to remove carbon contained in the compound, and sintering the compound from which the carbon is removed at a temperature of 700 to 1,000° C. to obtain a positive electrode active material including core particles having a composition of Formula 1 above and a shell composed of chromium oxide formed on a surface of the core particles.
Embodiments of the invention will be described in detail with reference to the accompanying drawings to the extent that those skilled in the art may readily practice. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.
In addition, in the drawings, anything unnecessary for describing the invention will be omitted for clarity, and also like reference numerals refer to like elements throughout.
Throughout the specification, the term “connected to” is used to designate a connection of one element to another element and include both a case where an element is “directly connected to” another element and a case where an element is “electronically connected to” another element via still another element.
Throughout the description, the term “on” that is used to designate a position of one element with respect to another element includes both a case that the one element is adjacent to the another element and a case that any other element is present between these two elements. Throughout the description, when an element “includes” a component, it may indicate that the element does not exclude another component unless explicitly described to the contrary, but can further include another component.
The terms “about”, substantially”, and the like used throughout the description indicates that when a natural manufacturing and a substance allowable error are suggested, such an allowable error corresponds the value or is similar to the value, and such values are intended for the sake of clear understanding of the invention or to prevent an unconscious infringer from illegally using the disclosure of the invention.
Throughout the description, the description of “A and/or B” indicates “A or B, or A and B”.
A positive electrode active material for a sodium ion battery according to the invention includes core particles having a composition of Formula 1 below, and a shell composed of chromium oxide formed on a surface of the core particles.
Na(Cr1-2xAl2x)O2 (x is 0.001 to 0.1) [Formula 1]
In the positive electrode active material according to the invention, when x is less than 0.001, a thickness of the formed Cr2O3 shell is not sufficient, and accordingly, cycle characteristics and resistance against water may be reduced, and when x is greater than 0.1, capacity is reduced. Therefore, the range of 0.001 to 0.1 is preferable. x may more preferably be 0.001 to 0.09, 0.001 to 0.08, 0.001 to 0.07, 0.0015 to 0.06, 0.002 to 0.05, 0.0025 to 0.004, 0.003 to 0.035, 0.0035 to 0.003, 0.004 to 0.0025, 0.0045 to 0.002, 0.005 to 0.018, 0.006 to 0.017, 0.007 to 0.016, 0.008 to 0.015.
In the positive electrode active material according to the invention, the chromium oxide may be Cr2O3.
In the positive electrode active material according to the invention, the core particles may be coated with the chromium oxide at a molar ratio of xCr2O3 (x is 0.001 to 0.03). x above may be 0.001 to 0.09, 0.001 to 0.08, 0.001 to 0.07, 0.0015 to 0.06, 0.002 to 0.05, 0.0025 to 0.004, 0.003 to 0.035, 0.0035 to 0.003, 0.004 to 0.0025, 0.0045 to 0.002, 0.005 to 0.018, 0.006 to 0.017, 0.007 to 0.016, 0.008 to 0.015.
In the positive electrode active material according to the invention, the shell may have a thickness of 1 to 30 nm. When the thickness of the shell is less than 1 nm, cycle characteristics and resistance against water may be reduced, and when the thickness of the shell is greater than 30 nm, capacity may be reduced. Therefore, the range of 1 to 30 nm is preferable. The thickness of the shell may be more preferably 1.5 to 25 nm, 2 to 20 nm, 2.5 to 10 nm, 3 to 8 nm, or 4 to 6 nm.
In the positive electrode active material according to the invention, the positive electrode active material may be inert to water (H2O) or moisture in the air.
A sodium ion battery according to the invention includes a positive electrode, a negative electrode disposed at a predetermined distance from the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte charged between the positive electrode and the negative electrode, and the separator, and the positive electrode includes a positive electrode active material including core particles having a composition of Formula 1 above and a shell composed of chromium oxide formed on a surface of the core particles.
The positive electrode may be formed to include a current collector and a positive electrode active material layer formed on the current collector. The current collector may include a metal current collector, for example, aluminum foil. The positive electrode active material layer may be prepared as a composition in the form of a mixture of a positive electrode active material powder, a conductive material, a binder, and a solvent, which is then molded and laminated on the metal current collector or applied onto the metal current collector to manufacture a positive electrode.
The negative electrode may be formed to include a current collector and a negative electrode active material layer formed on the current collector. The negative electrode active material layer may be prepared by applying a mixture prepared by mixing a negative electrode active material powder, a conductive material, a binder, and a solvent, directly onto a metal current collector and drying the resulting product, or casting a negative electrode active material composition onto a separate substrate, separating the composition from the substrate, and laminating the composition on a metal current collector. The negative electrode active material is not particularly limited as long as it is used in a sodium ion battery and is capable of reversible intercalation and de-intercalation of sodium ions.
The separator may preferably have a satisfactory electrolyte wettability while having low resistance against the movement of ions included in an electrolyte. The separator may be in the form of a nonwoven fabric or a woven fabric selected from, for example, glass fiber, polyester, Teflon, polyethylene, polypropylene, polytetrafluoroethylene (PTFB), or a combination thereof, and include polyethylene, polypropylene, and the like which are widely used in lithium-ion batteries.
The electrolyte is a non-aqueous electrolyte and may preferably be formed of an organic material, and a sodium salt may be dissolved in the organic material.
According to a method for preparing a positive electrode active material for a sodium ion battery includes dissolving a sodium (Na) precursor, a chromium (Cr) precursor, and an aluminum (Al) precursor in a solvent to prepare a solution, adding a chelating agent to the solution and stirring the mixture, drying the synthesized compound to obtain a compound containing sodium, chromium, and aluminum, heating the compound at a temperature of 400 to 600° C. to remove carbon contained in the compound, and sintering the compound from which the carbon is removed at a temperature of 700 to 1,000° C. to obtain a positive electrode active material including core particles having a composition of Formula 1 above and a shell composed of chromium oxide formed on a surface of the core particles.
In the method for preparing a positive electrode active material of the invention, the chelating agent may preferably be citric acid, but is not necessarily limited thereto.
In the method for preparing a positive electrode active material of the invention, the solvent may preferably be water, but is not necessarily limited thereto.
All chemicals (purity >99.5%) used in Examples of the invention were purchased from Merck. NaCrO2 and xCr2O3-coated Na(Cr1-2xAl2x)O2 (x=0.01, 0.025, and 0.5) were prepared through a sol-gel method using citric acid as a chelating agent.
NaNO3, Cr(NO3)3·9H2O, and Al(NO3)3·9H2O were dissolved in 50 ml of water at a molar ratio of 1:1:x to synthesize these materials. Then, citric acid was added to the solution, stirred at 70° C. for several hours, and then dried in a vacuum oven at 150° C. A black-green precursor thus produced was decarbonized in air at 500° C. for 6 hours, and then sintered at 900° C. in an Ar/H2 (5%) atmosphere to synthesize NaCrO2 and xCr2O3-coated Na(Cr1-2xAl2x)O2 powder.
In addition, for comparison, aluminum-doped Na(Cr0.95Al0.05)O2 and 0.025Cr2O3 chromium oxide-coated NaCrO2 were also synthesized through the same process as above. To prevent phase changes and surface reactions due to moisture contained in the atmosphere, all synthesized materials were stored in an argon-filled glove box.
In Examples of the invention, simply, an Al precursor was added in a stoichiometric ratio to a solution containing Cr and Na precursors. In this case, an amount of Al was changed to 2, 5, and 10 mol % of Cr to synthesize 0.01Cr2O3-coated NaCr0.98Al0.02O2 (hereinafter referred to as ‘1C-NCAO’), 0.025Cr2O3-coated NaCr0.95Al0.050O2 (hereinafter referred to as ‘2.5C-NCAO’), and 0.05Cr2O3-coated NaCr0.90Al0.10O2 (hereinafter referred to as ‘5C-NCAO’). Meanwhile, NaCO2 is indicated as ‘NCO’.
Whether an Al-doped NaCrO2 core and a Cr2O3 shell was spontaneously formed was investigated through the one-pot synthesis.
For the synthesized xC-NCAO (x=1, 2.5, and 5), Rietveld analysis was performed on the XRD pattern to determine whether the NCO structure was doped with Al.
Thereafter, changes in surface composition according to the addition of Al was investigated using XPS to determine that the synthesized compound was composed of a Cr2O3 shell and an NCAO core instead of a mixture of Cr2O3 and NCAO.
Referring to
Reflecting the core-shell structure formed in the synthesized material, the Al2p peak also showed a nonlinear behavior with the addition of Al.
For example, the Al2p peak intensity in 2.5C-NCAO increased only 1.12 times the Al2p peak intensity in 1C-NCAO although the amount of Al increased by 2.5 times.
The relative atom % of constituent elements and the formation of the NCAO core and Cr2O3 shell were also determined through STEM. EDX mapping shows a uniform distribution of Na, Cr, Al, and O with the atom % of 24.5±0.3% for Na, 24.4±0.2% for Cr, 0.6±0.2% for Al, and 50.5±0.9% for O, which is almost identical to the relative atomic ratio of 1C-NCAO. High-resolution TEM imaging was performed on an edge of particles to determine the presence of the Cr2O3 shell. As a result of TEM imaging, a clear dark contrast in which an about 5 nm thick bright surface layer is clearly visible was observed, and the electron diffraction pattern of a peripheral region showed that the surface layer was composed of Cr2O3 (
That is, it was determined that particles composed of the Al-doped NaCrO2 core and the Cr2O3 shell were synthesized through the above-described sol-gel method.
A positive electrode for electrochemical testing was prepared as follows.
First, the synthesized active material (80 wt %), carbon black (10 wt %, Super P), and polyvinylidene fluoride (10 wt %, HSV900, MTI) were mixed with N-methyl-2-pyrrolidone to prepare a positive electrode slurry. The prepared positive electrode slurry was applied onto aluminum foil and vacuum dried for 6 hours to prepare a positive electrode. The prepared positive electrode was assembled into a coin cell (CR2032, Wellcos) with metallic sodium as a negative electrode. An electrolyte of the coin cell was a mixture of ethylene carbonate and dimethyl carbonate (EC:DMC=1:1) containing 1.0 M NaPF6. All processes for the electrode preparation and the cell assembly were performed in an argon-filled glove box (O2, H2O<1 ppm).
Constant current charge/discharge cycle tests were performed using an automatic WBCS 3000 battery cycler (WonATech).
Electrochemical properties of xC-NCAO were tested during the constant current charge/discharge at a rate of 0.2 C (24 mA/g based on 1 C=120 mA/g) in a solution of 1.0 M NaPF6 using Na metal as a counter electrode. In addition, for comparison with the xC-NCAO characteristics, the same charge/discharge cycle tests were performed on NCO, 0.025Cr2O3-coated NCO (no Al doping), and Al-doped NaCr0.95Al0.05O2 (no Cr2O3 coating).
The fading rates of 0.025Cr2O3-coated NCO (no Al doping) and Al-doped NaCr0.95Al0.05O2 (no Cr2O3 coating) are −0.12 mAhg−1cycle−1 and −0.10 mAhg−1cycle−1, respectively, indicating that further improvement is needed.
Meanwhile, as determined in
In addition, 1C-NCAO has excellent rate characteristics, which is one of the favorable aspects of NCO. Despite the presence of a coating layer, the reversible capacity of 1C-NCAO did not significantly decrease from the high C-rate (
Positive electrode active materials for sodium ion batteries having a layered structure, such as NCO, are known to be sensitive to humid air, and water molecules may be inserted into an Na layer of the positive electrode active materials, causing expansion of the c-axis.
Powdered positive electrode active materials were exposed to air (humidity=about 3000 ppm) to evaluate moisture stability of the synthesized 1C-NCAO and NCO.
NCO and 1C-NCAO were exposed to air for 24 hours and then heated to 900° C. As determined in
In
NCO and 1C-NCAO powder were added to water as shown in
A positive electrode active material according to the invention exhibits excellent cycle stability and durability against water when applied to a positive electrode of a sodium ion battery.
According to a method for preparing a positive electrode active material according to the invention, a positive electrode active material having a core-shell structure, in which a shell is formed on a surface thereof to increase cycle stability and improve durability against humidity, may be prepared through a simple one-pot process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2023-0034016 | Mar 2023 | KR | national |
