Description:FIELD OF INVENTION

[0001] The present invention relates to a method of producing a cathode material for a sodium-ion battery and more particularly, to a method of synthesizing high performance polyanion cathode material for a sodium-ion battery.

BACKGROUND OF THE INVENTION

[0002] The subject matter discussed in the background section should not be assumed to be prior art merely because of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may correspond to implementations of the claimed technology.
[0003] Secondary battery systems are a promising technology for large scale energy storage due to its economic viability, high round trip efficiency, and ease of maintenance. The development of new types of high-performance energy storage and conversion technologies is urgently needed to meet the growing demands for portable electronic equipment, electric vehicles, and large-scale smart grids, and the like.
[0004] The need for advanced energy storage materials is growing, driven by the increasing demand for efficient, high-capacity batteries for various applications, including electric vehicles, renewable energy storage, and consumer electronics. In particular, sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs), offering advantages such as lower cost, greater natural abundance, and reduced environmental impact. Among the materials being explored for SIBs, sodium vanadium fluorophosphate (NVPF) has attracted considerable attention due to its favourable electrochemical properties, including high specific capacity, excellent cycle stability, and good rate capability. However, to maximize the electrochemical performance of NVPF-based cathodes, challenges remain, particularly regarding the synthesis method and electrode engineering. One of the most commercially viable and scalable approaches for NVPF cathode production is the solid-state synthesis method. Despite its advantages, this method faces significant challenges related to particle size control, aggregation of precursors, and achieving a uniform carbon coating, all of which can hinder the performance of the material.
[0005] NVPF has become a key candidate for use in sodium-ion batteries due to its unique structure and electrochemical characteristics. The development of NVPF materials has been extensively studied. Historically, several synthesis methods have been employed to fabricate NVPF-based cathodes, including the solid-state, sol-gel, hydrothermal, and mechanochemical routes. Of these, the solid-state synthesis method has been identified as the most commercially viable technique, particularly for bulk-scale production of NVPF cathodes. The solid-state method involves mixing a vanadium precursor, phosphate source, and carbon, followed by high temperature sintering to produce the NVPF/C composite. However, this method has limitations, especially in terms of particle size control, uniformity of the carbon coating, and the potential for precursor aggregation during the synthesis process.
[0006] In addition to the inherent synthesis challenges, the lack of a uniform carbon coating on the NVPF particles hinders their conductivity, which is essential for high-rate capability and overall performance. Several studies have explored the use of carbon precursors such as carbon soot or cellulose to create conductive coatings and to reduce vanadium and phosphate precursors to form NVPF. While these approaches have shown some success, they often face issues of particle aggregation, uneven coating, and insufficient conductivity, leading to suboptimal performance. Thus, the development of a simple, effective, and scalable approach to synthesize NVPF/C composites that can address these challenges is a key area of research.
[0007] There is therefore a need in the art for techniques capable of addressing the above-mentioned shortcomings by developing an environmentally friendly and cost-effective method for synthesizing NVPF/C composites with enhanced electrochemical performance.

OBJECTS OF THE INVENTION

[0008] An object of the present invention is to provide an efficient method for synthesizing high-performance sodium vanadium fluorophosphate (NVPF) cathode materials for sodium-ion batteries.
[0009] Another object of the present invention is to use high-surface-area functionalized carbon as both a reducing agent and conductive coating for NVPF.
[0010] Another object of the present invention is to prevent aggregation of NVPF particles during synthesis and ensure uniform dispersion and composition.
[0011] Another object of the present invention is to enhance the rate capability of NVPF/C composites, ensuring capacity retention at high discharge rates.
[0012] Another object of the present invention is to achieve high initial cycle coulombic efficiency and excellent cycling stability in NVPF/C composites.
[0013] Yet another object of the present invention is to enable large-scale production of NVPF/C composites suitable for commercial sodium-ion batteries.
[0014] Yet another object of the present invention is to enhance the dispersibility and uniformity of functionalized carbon during synthesis, improving the overall conductivity and stability of NVPF/C composite cathodes.

SUMMARY OF THE INVENTION

[0015] The summary is provided to introduce aspects related to a sodium-ion battery and a method for synthesizing a cathode material. Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.
[0016] An aspect of the present invention relates to a method for synthesizing NVPF/C composites that exhibit superior electrochemical performance for use in sodium-ion batteries. This method involves the use of high-surface-area functionalized carbon as both a reducing agent and a conductive coating on NVPF particles, ensuring better dispersion, uniformity, and conductivity. The resulting NVPF/C composite exhibits remarkable cycle stability, rate capability, and high reversible capacity, making it a promising candidate for large-scale applications in sodium-ion battery technology. Additionally, this method is environmentally friendly, economically viable, and scalable, making it a significant advancement in the production of high-performance cathode materials for sodium-ion batteries.
[0017] An aspect of the present invention relates to a method for synthesizing a cathode material Na3V2O2x (PO4)2F3-2x (0 ≤ x ≤ 1) for a sodium-ion battery that comprises the steps of utilizing a functionalized molecular carbon with surface functional groups as both a reducing agent and a source for forming a uniform conductive coating layer on the resulting cathode material, forming a stable dispersion of the functionalized carbon in a solvent, combining the dispersion with a vanadium source and a phosphate source to create a precursor mixture, processing the precursor mixture to obtain a carbon-coated vanadium phosphate intermediate, and treating the intermediate with a sodium and fluoride source under controlled conditions.
[0018] In another aspect of the present invention, the molecular carbon source comprises OH and/or -COOH and/or -SO4, and/or -NH-COOR and/or -NH2 and/or -COOR and/or -C-O-C- functional groups.
[0019] In another aspect of the present invention, the solvent is water or an organic and/or an inorganic solvent.
[0020] In another aspect of the present invention, the vanadium source is selected from the group consisting of vanadium pentoxide (V2O5) or any other vanadium containing compounds.
[0021] In another aspect of the present invention, the phosphate source is selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and/or their derivatives.
[0022] In another aspect of the present invention, the sodium and fluoride sources are selected from sodium fluoride and/or any other sodium and fluorine containing materials.
[0023] In another aspect of the present invention, it further comprises optimizing the molar ratios of the vanadium source, the phosphate source, the functionalized carbon, and the fluoride source.
[0024] In another aspect of the present invention, the processing includes steps of stirring, ball milling, drying, and thermal treatment under inert conditions at the temperatures between 500 °C to 1000 °C, preferably between 600 °C to 800 °C.
[0025] In another aspect of the present invention, the carbon content is between 1.0 % to 10.0 %, preferably ≤ 5.0 % of the total cathode material weight.
[0026] In another aspect of the present invention, the carbon coating forms a uniform surface layer on the cathode material with a thickness ranging from 3 nm to 15 nm, optionally greater than 5 nm and less than 12 nm, and preferably between 5 nm and 8 nm.
[0027] In another aspect of the present invention, the particle size distribution, on a volume basis, is as follows: Dv10 ≤ 2 μm, specifically ranging from 0.1 μm to 1.5 μm; Dv50 ≥ 1.0 μm, ranging from 1.0 μm to 10.0 μm, and preferably between 2 μm to 5 μm; and Dv90 ≥ 5.0 μm.
[0028] An aspect of the present invention relates to a sodium-ion battery comprises a cathode made from a cathode material synthesized, an anode made from a sodium-intercalating material, an electrolyte that facilitates ion transfer between the anode and cathode during charge and discharge cycles.
[0029] In another aspect of the present invention, the electrolyte is a liquid, gel, or solid-state electrolyte.
[0030] In another aspect of the present invention, the anode is made from hard carbon, sodium metal, and/or sodium-intercalating, alloying materials.
[0031] In another aspect of the present invention, it further comprises a separator positioned between the anode and cathode.
[0032] Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0033] The accompanying drawings constitute a part of the description and are used to provide further understanding of the present invention. Such accompanying drawings illustrate an embodiment of the present invention which is used to describe the principles of the present invention together with the description. The foregoing aspects and many of the advantages of this invention will become more readily appreciated as the same becomes better understood by the reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:
[0034] Fig. 1 illustrates a schematic illustration of NVPF/C preparation, in accordance with an embodiment of the present invention;
[0035] Fig. 2 illustrates powder x-ray diffraction of synthesized NVPF/C, in accordance with an embodiment of the present invention;
[0036] Fig. 3 illustrates High Resolution Transmission Electron Microscopy (HR-TEM) image of synthesized NVPF/C, in accordance with an embodiment of the present invention;
[0037] Fig. 4 illustrates the volume distribution of particles of synthesized NVPF/C, in accordance with an embodiment of the present invention;
[0038] Fig. 5a and 5b illustrate a half-cell configuration employing sodium (Na) metal as anode, in accordance with an embodiment of the present invention;
[0039] Fig. 6a and 6b illustrate full cell configurations employing hard carbon anode, in accordance with an embodiment of the present invention;

DETAILED DESCRIPTION OF THE INVENTION

[0040] The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.
[0041] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0042] The terms “or” and “and/or” as used herein are to be interpreted as inclusive or meaning any one or any combination. Therefore, “A, B or C” or “A, B and/or C” mean “any of the following: A; B; C; A and B; A and C; B and C; A, B and C.” An exception to this definition will occur only when a combination of elements, functions, steps or acts are in some way inherently mutually exclusive.
[0043] The present invention introduces a method for synthesizing high-performance NVPF/C composite cathode materials for sodium-ion batteries. This method utilizes functionalized carbon with -OH and -COOH groups, which serves dual purposes as both a reducing agent and a conductive coating on NVPF particles. The process involves dispersing the carbon in water, mixing it with vanadium pentoxide and phosphate precursors, followed by ball milling and heat treatment to form VPO4/C. The addition of NaF in the second step ensures the formation of pure NVPF/C material. This approach prevents particle aggregation, facilitates uniform carbon coating, and optimizes the electrochemical properties of the cathode, resulting in high capacity, excellent rate capability, and long-term cycling stability.
[0044] Fig. 1 illustrates utilizing a functionalized carbon as both a reducing agent and conductive coating layer on the NVPF cathode material. This helps in preventing the aggregation of precursors and resultant products. The molar ratios of VPO4/C and NaF, reaction temperature and reaction time are optimized to produce phase pure NVPF/C cathode material without incurring any fluorine deficiency. The functionalized carbon used may include, but are not limited to, either -OH and/or -COOH and/or -SO4, and/or -NH-COOR and/or -NH2 and/or -COOR and/or -C-O-C- functional groups. The at least one of functionalized carbon, that is, -OH and/or -COOH and/or -SO4, and/or -NH-COOR and/or -NH2 and/or -COOR and/or -C-O-C- functional groups is used as both reducing agent and a conductive coating layer on NVPF. A predetermined amount of functionalized carbon is dispersed in de-ionized water using a homogenization technique, such as, but not limited to, ultrasonication, for a predetermined amount of time, preferably thirty minutes. Ultrasonication, also known as sonication, is a homogenization technique that uses high-frequency sound waves (above 20 kHz) to break down large particles into smaller, more uniform fragments within a fluid.
[0045] In a preferred embodiment of the present invention, optimized molar ratios of 1 mmol V₂O₅, 2 mmol NH₄H₂PO₄, were added to carbon (2.5 mmol) dispersed deionized water and stirred for 3 hours to ensure uniform mixing. The resulting slurry type mixture was then dried for 12 hrs in a vacuum oven at 100 oC, and the solid residue was ball milled for 1 hour. The finely milled powder was subsequently heat-treated at 700 °C for 8 hours under an argon atmosphere to obtain VPO₄/C in the first step. In the second step, 2 mmol of the previously synthesized VPO₄/C was mixed with 3 mmol of NaF (including an additional 5 wt% excess NaF), milled for 2 hrs, and heat-treated at 650 °C for 8 hours to synthesize NVPF/C. Soluble impurities were removed by washing the product with deionized water and ethanol, followed by drying at 100 °C for 8 hrs to obtain the final purified NVPF/C material.
[0046] Further, a predetermined amount of vanadium pentoxide (V2O5) and either ammonium dihydrogen phosphate or diammonium hydrogen phosphate are introduced in the carbon dispersion process. This mixture is then stirred for a predetermined amount of time, preferably three hours. The obtained slurry-type reaction mixture is dried at a predetermined temperature, preferably at least at 100 OC in a vacuum oven for a predetermined amount of time, that is, at least, twelve hours. The dried solid is subjected to milling, such as, ball milling and the like. Then the finely milled powder was subsequently heat-treated at 700 °C for 8 hours under an argon atmosphere to obtain VPO₄/C intermediate in the first step. A stoichiometric amount of VPO4/C (2 mmol) and NaF (3 mmol preferably 5 wt% extra) were taken and grounded using a ball mill for at least 2 hours. The reaction mixture is dried and subsequently calcinated at a predetermined temperature, preferably at least, 650°C for a predetermined time, preferably, 8 hours in an inert atmosphere. The resulting NVPF/C powder is then washed with water and ethanol, followed by drying at a predetermined temperature, preferably, 100°C in an air oven for a predetermined time, preferably at least, 8 hours. The dried powder is grounded well, and cathode electrodes are produced using the Applicant’s aqueous slurry process by taking 90 wt%, 7wt %, 1 wt% and 2wt% of NVPF/C,C65, CMC and PAA respectively The typical loading of cathode active material is maintained at preferably 8-10 mg/cm2.
[0047] Fig. 2 illustrates powder x-ray diffraction of synthesized NVPF/C
The crystal system or the crystal structure is preferably Tetragonal and Space group is preferably P42/mnm. This is achieved since the method for synthesizing a cathode material Na3V2O2x (PO4)2F3-2x (0 ≤ x ≤ 1) for a sodium-ion battery involves utilizing a functionalized molecular carbon with surface functional groups as both a reducing agent and a source for forming a uniform conductive coating layer on the resulting cathode material. It is then made to form a stable dispersion of the functionalized carbon in a solvent. The dispersion is combined with a vanadium source and a phosphate source to create a precursor mixture. This precursor mixture is processed to obtain carbon-coated vanadium phosphate intermediate which is then treated with a sodium source and/or a fluoride source under controlled conditions. the molecular carbon source may be either of -OH and/or -COOH and/or -SO4, and/or -NH-COOR and/or -NH2 and/or -COOR and/or -C-O-C- functional groups. The solvent used may be water or an organic or an inorganic solvent.
[0048] The vanadium source is selected from a group containing vanadium pentoxide (V2O5) or from any other vanadium containing compounds. Whereas, the phosphate source is selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and/or their derivatives. Further, the sodium and fluoride sources are selected from sodium fluoride and/or any other sodium and fluorine containing materials. The molar ratios of the vanadium source, the phosphate source, the functionalized carbon, and the fluoride source are optimized in accordance with the predetermined criterions.
[0049] The processing steps may include either of or all of stirring, milling, preferably ball milling, drying and thermal treatment under inert conditions at temperatures between 500°C to 1000°C, preferably between 600°C to 800°C. The carbon content is kept between 1.0 % to 10.0 %, preferably ≤ 5.0 % of the total cathode material weight.
[0050] Fig. 3 illustrates the High-Resolution Transmission Electron Microscopy (HR-TEM) image of NVPF/C material produced in accordance with an embodiment of the present invention. The HR-TEM analysis confirmed a uniform carbon coating of approximately 6 nm thickness on the surface of the NVPF particles.
[0051] Fig.4 illustrates the particle size distribution of the NVPF/C material synthesized according to an embodiment of the present invention. The distribution, based on volume, is maintained at Dv10: 0.8 µm, Dv50: 2.9 µm, and Dv90: 7.9 µm.
[0052] Fig. 5a and 5b illustrate a half-cell configuration employing sodium (Na) metal as anode to assess the electrochemical performance of NVPF/C cathode. The battery comprises a cathode made from a cathode material synthesized by the process as described, an anode made from the metallic sodium, and an electrolyte that facilitates ion transfer between the anode and cathode during charge and discharge cycles. The electrolyte is a liquid comprised with propylene carbonate solvent and 1 M NaClO4 salt.. The half-cell exhibits an initial cycle coulombic efficiency of 95%, along with a high reversible capacity of 120 mAh/g at a rate of C/20. Notably, the material showcased remarkable rate capability, retaining over 90% of its capacity at 2C discharge rate, and demonstrated impressive cycling stability over prolonged charge-discharge cycles.
[0053] The anode is made from hard carbon, sodium metal, and/or sodium-intercalating, alloying materials. It also includes a separator positioned between the anode and cathode. The configuration discloses the relationship between the potential and specific capacity along with relationship between the specific capacity and cycle number.
[0054] Fig. 6a and 6b illustrate electrochemical performance of the NVPF/C material produced in accordance with an embodiment of present invention in the full cell configuration employing hard carbon as anode. It relates to a relationship between specific capacity and voltage along with a relationship between specific capacity and cycle number. The full cell exhibits an initial coulombic efficiency of 83% with a reversible specific capacity of 102 mAh/g at a rate of C/20, which is pivotal in determining the cell energy density. Besides, the cell displayed an excellent rate capability up to 2C discharge rate and cycling stability at 1C charge and 2C discharge rates.
[0055] In view of the present disclosure which describes the present invention, all changes, modifications and variations within the meaning and range of equivalency are considered within the scope and spirit of the invention. It is to be understood that the aspects and embodiment of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiment may be combined together to form a further embodiment of the disclosure.
, Claims:1. A method for synthesizing a cathode material Na3V2O2x (PO4)2F3-2x (0 ≤ x ≤ 1) for a sodium-ion battery, comprising the steps of:
utilizing a functionalized molecular carbon with surface functional groups as both a reducing agent and a source for forming a uniform conductive coating layer on the resulting cathode material;
forming a stable dispersion of the functionalized carbon in a solvent;
combining the dispersion with a vanadium source and a phosphate source to create a precursor mixture;
processing the precursor mixture to obtain a carbon-coated vanadium phosphate intermediate, and
treating the intermediate with a sodium and fluoride source under controlled conditions.

2. The method as claimed in claim 1, wherein the molecular carbon source comprises OH and/or -COOH and/or -SO4, and/or -NH-COOR and/or -NH2 and/or -COOR and/or -C-O-C- functional groups.

3. The method as claimed in claim 1, wherein the solvent is water or an organic and/or an inorganic solvent.

4. The method as claimed in claim 1, wherein the vanadium source is selected from the group consisting of vanadium pentoxide (V2O5) or any other vanadium containing compounds.

5. The method as claimed in claim 1, wherein the phosphate source is selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and/or their derivatives.

6. The method as claimed in claim 1, wherein the sodium and fluoride sources are selected from sodium fluoride and/or any other sodium and fluorine containing materials.

7. The method as claimed in claim 1, further comprising optimizing the molar ratios of the vanadium source, the phosphate source, the functionalized carbon, and the fluoride source.

8. The method as claimed in claim 1, wherein the processing includes steps of stirring, ball milling, drying, and thermal treatment under inert conditions at the temperatures between 500 °C to 1000 °C, preferably between 600 °C to 800 °C.

9. The cathode material as claimed in claim 1, wherein the carbon content is between 1.0 % to 10.0 %, preferably ≤ 5.0 % of the total cathode material weight.

10. The cathode material as claimed in claim 1, wherein the carbon coating forms a uniform surface layer on the cathode material with a thickness ranging from 3 nm to 15 nm, optionally greater than 5 nm and less than 12 nm, and preferably between 5 nm and 8 nm.

11. The cathode material as claimed in claim 1, wherein the particle size distribution, on a volume basis, is as follows: Dv10 ≤ 2 μm, specifically ranging from 0.1 μm to 1.5 μm; Dv50 ≥ 1.0 μm, ranging from 1.0 μm to 10 μm, and preferably between 2 μm to 5 μm; and Dv90 ≥ 5 μm.

12. A sodium-ion battery comprising:
a cathode made from a cathode material synthesized by the method as claimed in claim 1;
an anode made from a sodium-intercalating material;
an electrolyte that facilitates ion transfer between the anode and cathode during charge and discharge cycles.

13. The battery as claimed in claim 7, wherein the electrolyte is a liquid, gel, or solid-state electrolyte.

14. The battery as claimed in claim 7, wherein the anode is made from hard carbon, sodium metal, and/or sodium-intercalating, alloying materials.

15. The battery as claimed in claim 7, further comprising a separator positioned between the anode and cathode.