Patent Application
20190148729
The disclosure of the present patent application relates to sodium-ion batteries, and particularly to an electrode for a sodium-ion battery that is a fluorinated sodium metal phosphate compound that can be used in a positive electrode for a rechargeable sodium-ion battery.
Lithium-ion rechargeable batteries have been commercially available for several years. However, lithium metal is a scarce resource, and with demand for lithium-ion batteries constantly increasing, the price of lithium has been steadily increasing. Consequently, there is renewed interest in developing a sodium-ion battery, since the two elements have similar properties, but sodium is cheaper and more readily available. In one important respect, however, sodium is different from lithium, viz., sodium is a larger atom than lithium. The effect of this difference in size is that sodium ions are not transported through electrolyte as quickly as lithium ions, causing a slower response to a sudden demand for current. Hence, some of the technology developed for lithium electrodes and electrodes does not carry over directly to electrodes and electrolytes for sodium-ion batteries. There is a need for developing electrodes and electrolytes having properties consistent with their use in sodium-ion batteries.
Thus, an electrode for a sodium-ion battery solving the aforementioned problems is desired.
The electrode for a sodium-ion battery is a fluorinated sodium metal phosphate having the general formula Na3+xV2−xMx(PO4)2F3, wherein “M” is a divalent metal selected from the group consisting of Mg, Cr, Mn, Fe, Co, Ni, and Cu and 0<x≤1. Materials comprising such compounds can be used as positive electrode materials for rechargeable sodium-ion batteries. The compounds of the present disclosure may be produced by a hydrothermal or a solid-state synthesis route.
These and other features of the present disclosure will become readily apparent upon further review of the following specification.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
The electrode for a sodium-ion battery is a fluorinated sodium metal phosphate having the general formula Na3+xV2−xMx(PO4)2F3, wherein “M” is a divalent metal selected from the group consisting of Mg, Cr, Mn, Fe, Co, Ni, and Cu and 0<x≤1. The compound from which the electrode is made is preferably in a solid-state form.
The compounds of the present disclosure may be made by hydrothermal or solid-state synthesis, as described in the following examples.
The Na3+8V2−xMx(PO4)2F3 compounds [wherein “M” is a divalent cation that can be chosen from, but is not limited to, Mg, Cr, Mn, Fe, Co, Ni, Cu, and (0<x≤1)] were successfully prepared using a hydrothermal method from stoichiometric mixtures of NaF (Aldrich, ≥99%), NH4VO3 (Aldrich, ≥99.99%), M(CH3COO)3.xH2O, (Aldrich, ≥99.99%), NH4H2PO4 (Aldrich, 99.99%) and citric acid (C6H8O7) (CA). CA was employed as carbon source and reducing agent. First NH4VO3 and CA with a mole ratio of 1:2 were dissolved in 40 ml of water to form a clear blue solution, and then M(CH3COO)3.xH2O was added (Solution A). The NaF and NH4H2PO4 were dissolved together in 40 ml of H2O (Solution B). Solution B was then added dropwise to solution A under continuous stirring. The solution is finally poured in a 100 mL autoclave, which was then heated at 200° C. for 20 h. The powder obtained after filtering the solution was dried at 100° C. for 12 h under vacuum. The progress of the reaction was followed by PXRD.
The Na3+xV2−xMx(PO4)2F3 compounds [wherein “M” is a divalent cation that can be chosen from, but is not limited to, Mg, Cr, Mn, Fe, Co, Ni, Cu, and (0<x≤1)] were also successfully prepared using a sol-gel method from stoichiometric mixtures of NaF (Aldrich, ≥99%), NH4VO3 (Aldrich, ≥99.99%), NH4H2PO4 (Aldrich, 99.99%) and citric acid (C6H8O7) (CA). CA was employed as carbon source and reducing agent. First NH4VO3 and CA were dissolved in 100 ml of water to form a clear blue solution (Solution A). M(CH3COO)3xH2O (Aldrich, ≥99.99%) is dissolved in 50 ml of water and then added to Solution A. The NaF and NH4H2PO4 were mixed together under continuous stirring in 50 ml of H2O (Solution B). Solution B was then added dropwise to solution A under continuous stirring. The resulting solution was then slowly evaporated to dryness at 100° C. The residue was ground in an agate mortar and heated in Ar atmosphere in an alumina crucible at 400° C. for 24 h and at 650° C. for 24 h.
In the above syntheses, the precursors for the synthesis can also be replaced as follows: (1) NH4VO3 may be replaced by VOSO4, VCl3.xH2O, VOC2O4, V2O5, V2O3, or VO2; (2) M(CH3COO)3.xH2O may be replaced by MSO4,xH2O, M(NO3)2.xH2O, or MCl2.xH2O; (3) NH4H2PO4 may be replaced by (NH4)2HPO4, H3PO4, Na2HPO4, or NaH2PO4; (4) NaF may be replaced by (NH4)2F, HF, or MF; and (5) the reducing agent, (RA) is not limited to citric acid (C6H8O7) (CA), but may be replaced by oxalic acid H2C2O4 (OA), Formic acid (HCOOH) or maleic acid C4H4O4.
To ensure the purity of the Na3+xV2−xMx(PO4)2F3 powders, PXRD measurements were performed. The data were collected at room temperature over the 2θ angle range of 10°≤2θ≤70° with a step size of 0.01° using a Bruker d8 Avanced diffractometer operating with CuKα radiations. Full pattern matching refinement was performed with the Jana2006 program package. The background was estimated by a Legendre function, and the peak shapes were described by a pseudo-Voigt function. An exemplary diffractogram for the electrode material of formula Na3.2V1.8Ni0.2(PO4)2F3 is shown in
Based on the full pattern matching performed on all the Na3+xV2−xMx(PO4)2F3 samples, the powder patterns could be indexed either using the space group I4/mmm or P42/mnm. This indicates that the crystal structures of our compounds are either isostructural to Na3Cr2(PO4)2F3 or Na3V2(PO4)2F3, respectively. The main difference between the two structures is the distribution of the sodium atoms within the [V2(PO4)2F3]3− frameworks, and also the slight distortion of the octahedra containing the vanadium cations. It is worthwhile to mention that all the Na3M2(PO4)2F3−yOy, also have very similar [M2(PO4)2F3]3− frameworks, even though they crystallize with different space groups (I4/mmm, P42/mnm, P42/mbc, Cmcm, Cmc21, or Pbam).
SEM images of exemplary samples of the synthesized electrode material are shown in
Positive electrodes were made from mixtures of Na3+xV2−xMx(PO4)2F3 powders, acetylene black (AB) and polyvinylidene fluoride (PVDF) in a weight ratio of 80:10:10. The resulting electrode film was pressed with a twin roller, cut into a round plate (Φ=14 mm), and dried at 120° C. for 12 h under vacuum. The electrolyte was 1 M NaPF6 dissolved in ethylene carbonate (EC) and propylene carbonate (PC) [EC/PC with 1/1 in volume ratio]. Coin-type cells (CR2032) embedding Na3+xV2−xMx(PO4)2F3/NaPF6+EC+PC/Na were assembled in an argon-filled glove box with a Whatman GF/C glass fiber separator. Room temperature galvanometric cycling tests (Constant current mode) were performed using Arbin battery tester system in a potential range of 2.5-4.5 V at different rates, whereas the cyclic voltammetry tests were performed using a Solartron battery tester system. All our electrochemical tests were performed in half cells versus Na metal anode.
The electrolyte salts may be selected from, but are not limited to, the group consisting of NaPF6, NaClO4, and NaBF4. The carbonate solvent may be selected from, but is not limited to, the group consisting of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).
The Galvanostatic charge and discharge curves shows that Na3.2Ni0.2V18(PO4)2F3 delivers a discharge capacity of 115 mAh/g with excellent cycling performance after 500 cycles (
The Galvanostatic charge and discharge curves shows that Na3.5Ni0.5V1.5(PO4)2F3 delivers a discharge capacity of 84 mAh/g at 1 C rate, as shown in
Thus, the materials and the compounds may provide electrochemical energy storage of sodium ions by functioning as, for example, positive electrodes for sodium-ion batteries.
A device, typically a battery, may be made with a positive electrode formed from the material or the compound and a negative electrode formed from hard carbon, and further having a sodium-based electrolyte.
It is to be understood that the electrode for a sodium-ion battery is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/586,796, filed Nov. 15, 2017.
| Number | Date | Country | |
|---|---|---|---|
| 62586796 | Nov 2017 | US |
