Biomass-derived carbon as electrode for sodium ion battery, and process of preparation thereof

Abstract

The present disclosure relates to a biowaste-derived carbon and their use in energy storage applications. Specifically, the present disclosure relates to a hard carbon prepared from legumes of trees in the family of royal poinciana (Gulmohar) for use as electrode in energy storage devices. The present disclosure also relates to a method of preparing the hard carbon from Gulmohar by a single-step process.

Description

[0001] BIOMASS-DERIVED CARBON AS ELECTRODE FOR SODIUM ION BATTERY, AND PROCESS OF PREPARATION THEREOF

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present disclosure relates to a biowaste-derived carbon and their use in energy storage applications. Specifically, the present disclosure relates to a hard carbon prepared from legumes of trees in the family of royal poinciana (Gulmohar) for use as electrode in energy storage devices. The present disclosure also relates to a method of preparing the hard carbon from Gulmohar by a single-step process.

[0004] BACKGROUND AND PRIOR ART OF THE INVENTION

[0005] The key parameter in development of sodium ion battery (SIB) is the electrode materials with high performance. Carbon materials are widely researched for the application of energy storage devices. Particularly, due to higher d-spacing value and disordered structure, hard carbon (HC) is a suitable choice for anode in SIB. HC is prepared by pyrolyzing thermosetting carbon containing precursor particularly biowaste in an inert atmosphere at higher temperatures. HC prepared from biowaste majorly contains C with small quantity of N and O. N and O presence in HC is beneficial for better electronic and electrochemical properties.

[0006] Many types of biowastes are used for hard carbon synthesis (coconut shell, agri waste, and the like) however these are not found to be uniform in properties when collected from different sources. It is therefore necessary to obtain carefully cultivated bioresources to obtain uniform hard carbon for sodium batteries to avoid performance variation.

[0007] In certain hard carbon synthesis processes employing biowaste as a precursor, various pretreatments were conducted to manipulate surface area, porosity distribution, or heteroatom doping of N / P / O / S, among other factors. Additionally, elevated synthesis temperatures up to 1300 °C were employed for hard carbon synthesis. While these approaches yield hard carbon materials with improved performance, they entail considerable expenses associated with precursors such as acid / alkali treatment sources, heteroatom dopant resources, and elevated energy consumption during high-temperature treatment.

[0008] Royal poinciana (Gulmohar) is very common in India and produces flat strapped pods. The pod waste without seeds goes into landfill. The peculiar spongy structure of the legume has found to be beneficial for hard carbon (HC) prepared with N and O doping, higher d-spacing with good electronic conductivity.

[0009] Chen et al. disclose about the fast microwave-assisted approach was developed to fabricate carbon microspheres (CSs) using sucrose as the precursor in a microwave system at 500 °C in argon atmosphere (J. Mater. Chem. A, 2014, 2, 1263-1267). Said CSs have a specific capacity of 183 mAhg-1, Current density of 30 mA g1, and Cycle number of 50.

[0010] Zhang et al., discloses a method for producing N and O dual-doped carbon microspheres (NO- CS) from com stalks. Said process involves pre-treatment of washing HNO3 and H2O2 from corn stalks. The method further involves autoclaving at 180 °C for 48 hours and carbonization at 1200 °C for 2 hours with 10 °C min1heating rate (Electrochimica Acta Volume 303, 20 April 2019, Pages 140-147). The NO-CS derived carbon anode exhibited tremendous reversible capacity, rate capability and cycling stability, e.g. 270 mAh g-1over 200 cycles at 50 mA g-1, 130 mAh g-1over 3500 cycles at 5 A g-1. However, the method involved too many steps to arrive at the NO-CS derived carbon that involved wet- / dry carbonization and solution doping route from the bio waste of corn stalks.

[0011] ACS Appl. Energy Mater. 2020, 3, 10, 10045-10052 discloses Hard Carbon Microsphere (HCMS) derived from sucrose using Micro-wave assisted approach and pyrolysis at 1000 °C in argon atmosphere. However, the method involved is not a single step process and resulted in HCMS having a Specific capacity of 300 mAhg1, Current density of 30 mA g1, and Cycle number of 50.

[0012] Accordingly, the present disclosure provides a cost-effective process of producing robust hard carbon material from royal poinciana pods (Gulmohar) which avoids acid / alkali pre- or posttreatments and also, covers lesser temperature of pyrolysis around 1000 °C which is economically significant considering the bulk and industry level production.

[0013] OBJECTIVES OF THE INVENTION

[0014] Accordingly, the main objective of the present disclosure is to provide a cost-effective, environmental friendly process to prepare robust hard carbon material for use as an electrode for energy storage applications. An objective of the present disclosure is to provide a straightforward one-step preparation of hard carbon from biowaste, wherein the process does not involve any pre / post-treatment.

[0015] An objective of the present disclosure is to provide a biowaste-derived hard carbon (HC) for use as an electrode in high energy storage devices.

[0016] Another objective of the present disclosure is to provide an energy storage device comprising the Gulmohar pod-derived HC as an electrode.

[0017] Yet another objective of the present disclosure is to provide a sodium- ion cells / batteries comprising the Gulmohar pod-derived HC as an electrode.

[0018] SUMMARY OF THE INVENTION

[0019] Aspects of the present disclosure relates to a biowaste-derived carbon and their use in energy storage applications. Specifically, the present disclosure relates to a hard carbon (HC) prepared from legumes of trees in the family of royal poinciana (Gulmohar) for use as electrode in energy storage devices. The present disclosure also relates to a method of preparing the HC from Gulmohar by a single-step process.

[0020] In an aspect, the present disclosure provides a hard carbon as electrode for energy storage device, comprising: nitrogen and oxygen doping and a chunk like morphology with interlayer spacing; wherein the hard carbon is a bio-waste derived hard carbon, and has a surface area ranging from 115 to 130 m2g1.

[0021] In an embodiment, the bio-waste comprises pods of waste Royal poinciana (Gulmohar pods).

[0022] In another aspect, the present disclosure provides a process for preparing the hard carbon (HC), comprising the steps of: a) processing a bio-waste to remove impurities; and b) pyrolizing the processed bio-waste under an inert atmosphere followed by powdering to obtain the hard carbon.

[0023] In an embodiment, the bio-waste comprises pods of waste Royal poinciana.

[0024] In an embodiment, the processing at step a) comprises washing of the bio-waste with a solvent mixture comprising of ethanol and water to remove impurities, removing moisture by drying the washed bio-waste in an oven at temperature in the range of 80-100 °C, and cutting the dried bio-waste into uniform pieces.

[0025] In an embodiment, the pyrolizing at step b) is carried out at temperature in the range of 900- 1100 °C for a time period in the range of 2-4 hours with a heating rate of 5 °C / min.

[0026] In an embodiment, the inert atmosphere is composed of argon gas or nitrogen gas.

[0027] In an embodiment, the process of preparation of HC is free of post-acid treatment or post-alkali treatment.

[0028] In another aspect, the present disclosure provides a process for preparing an electrode using the hard carbon, comprising the steps of: a. mixing the hard carbon (HC), a conducting additive, and a binder into a solvent to obtain a homogenous slurry; and b. depositing the homogenous slurry on a conducting foil configured to act as a current collector followed by drying for time period in the range of 8-12 h to obtain the electrode.

[0029] In an embodiment, the bio-waste comprises pods waste of Royal Poinciana.

[0030] In an embodiment, the hard carbon, conducting additive, and binder are present in a ratio ranging from 60: 10:5 to 80:30: 15.

[0031] In an embodiment, the drying at step b) is carried out at a temperature ranging from 60 to 80 °C.

[0032] In another aspect, the present disclosure provides an energy storage device comprising the hard carbon as a working electrode.

[0033] In an embodiment, the energy storage device is selected from a sodium-ion battery, a sodium metal battery, a lithium-ion battery, a lithium metal battery, and lithium-sulfur (Li-S) battery.

[0034] In an embodiment, the energy storage device additionally comprises: i) negative and positive casings (1), ii) sodium metal (2), iii) a separator wet or in contact with electrolyte (3), iv) working electrode coated with the hard carbon (4), v) a spacer (5), and vi) a spring (6).

[0035] DETAILED DESCRIPTION OF THE DRAWINGS The invention has other advantages and features which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0036] Figure 1 depicts depict (a) XRD, (b) N2 adsorption-desorption isotherm and (c) Pore size distribution, (d-f) FESEM images of hard carbon prepared from royal poinciana pods.

[0037] Figure 2 depicts depict the XPS analysis of hard carbon, (a) XPS survey, (b) C Is, (c) N Is, (d) O ls, and (e) Si 2p deconvoluted peaks.

[0038] Figure 3 depicts the (a) CV curves of hard carbon at 0.1 mV s1scan rate, (b) cycling stability at 100 mA g1with charge-discharge curves for some cycles (c), (d) rate performance at varied current densities, (e) stability at 25 mA g1current density with charge-discharge curves for initial cycles (f).

[0039] Figure 4 depicts the coin cell containing the hard carbon disclosed in present disclosure.

[0040] Figure 5 shows the cycling stability of sodium ion full cell (HC / / NVP) containing the hard carbon as anode at 100 mA g1.

[0041] SOURCE OF BIOLOGICAL MATERIAL:

[0042] The present invention involves waste flat-strapped pods without seeds from royal poinciana trees found locally in Pune, Maharashtra, India.

[0043] DETAILED DESCRIPTION OF THE INVENTION:

[0044] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0045] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0046] The tables, figures and protocols have been represented where appropriate by conventional representations in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

[0047] The terms “bio-waste derived hard carbon (HC)” or “gulmohar pod-derived hard carbon” or “gulmohar pod-derived HC” or “gulmohar derived HC” or “hard carbon” are used herein interchangeably with same meaning throughout the specification.

[0048] The post-acid treatment term used herein means treated the obtained product (in this HC) is treated with acid solution.

[0049] The post-alkali treatment term used herein means treated the obtained product (in this HC) is treated with base solution.

[0050] Accordingly, embodiments of the present disclosure relate to a biowaste-derived carbon and their use in energy storage applications. Specifically, the present disclosure relates to a hard carbon prepared from legumes of trees in the family of royal poinciana (Gulmohar) for use as electrode in energy storage devices. The present disclosure also relates to a method of preparing the hard carbon from Gulmohar by a single-step process.

[0051] In an embodiment of the present disclosure, the gulmohar pod-derived hard carbon contains nanopores in order to achieve the required electrochemical properties for use in energy storage devices.

[0052] In some embodiments of the present disclosure, the nanopores present in the gulmohar pod- derived hard carbon have a diameter of at least about 3 nm. In some embodiments, the nanopores may have a diameter of less than 20 nm. The diameter of nanopores can range from any of the minimum values described above to any of the maximum values described above, for example from 3 nm to 20 nm.

[0053] In an embodiment, the present disclosure aims to generate high levels of nanoporosity while simultaneously preserving the N and O content of the precursor to achieve optimum Na-ion storage capability. In some embodiments, this may be achieved by minimizing (relative to previous studies) the temperature and time at which carbonization / activation is performed.

[0054] In an aspect, the present disclosure provides a bio-waste derived hard carbon (HC), including nitrogen and oxygen doping and a chunk like morphology with interlayer spacing (d-spacing); the bio-waste derived hard carbon (HC) has a surface area ranging from 115 to 130 m2g1. The bio- waste comprises pods waste of royal poinciana (Gulmohar pods).

[0055] The surface area of hard carbon may addresses issues like increased electrolyte consumption and low initial columbic efficiency i.e., the hard carbon disclosed herein cover optimum electrolyte consumption (not too low or too high), and has higher initial columbic efficiency.

[0056] In an embodiment, a spongy structure of the bio waste material is found to be beneficial for HC prepared with N and O doping, and higher d-spacing with good electronic conductivity.

[0057] In an embodiment, the nitrogen and oxygen doping improves ion and electron transferability and create more active sites for sodium ion storage in case of the sodium ion battery. The same would also be applicable to other types of the batteries mentioned in the specification.

[0058] In an embodiment of the present disclosure, the gulmohar pod-derived hard carbon is used in the production of electrodes for energy storage devices. In some embodiments, the electrode is a cathode, an anode or both. Preferably, an anode.

[0059] In an embodiment of the present disclosure, the energy storage device is a battery, selected from but not limited to Na-ion battery, a Na metal battery, a Li-ion battery or a Li metal battery, Lithium-Sulfur (Li-S) battery. Preferably, a Na-ion battery.

[0060] In an embodiment of the present disclosure, the gulmohar pod-derived HC exhibits N and O doping, higher d-spacing with good electronic conductivity. N and O presence in HC is beneficial for better electronic and electrochemical properties.

[0061] In another embodiment of the present disclosure, the gulmohar pod-derived HC exhibits an exceptional electrochemical performance for Na+ion storage, at a current rate of 0.025 A g1, exhibiting a specific capacity of 222 mAh g1. In another embodiment of the present disclosure, the gulmohar pod-derived HC exhibits a chunklike morphology, lacking a distinct, specific structure. The surface presents a rough texture with the appearance of particle-like entities.

[0062] In another embodiment of the present disclosure, the gulmohar pod-derived HC exhibits a specific surface area of 100-150 m2g1. For example, 100-110 m2g1, 110-120 m2g1, 120-130 m2g1, 130-140 m2g1, 140-150 m2g1. Preferably 120-130 m2g1and more preferably 121 m2g1-

[0063] In still another embodiment of the present disclosure, the gulmohar pod-derived HC comprises C, N and O, with any impurity elements being lower than 1.5 - 2 wt %. The gulmohar pod- derived HC comprises graphitic and pyrrolytic nitrogen, and in traces, Silicon (Si), wherein nitrogen (N) and silicon (Si) content derived from the natural precursor of gulmohar pods. In some embodiments, the gulmohar pod-derived hard carbon comprises 1.5 - 2% of nitrogen (N) and 1.5 - 2% of silicon (Si).

[0064] In yet another embodiment, the present disclosure provides a straightforward one-step synthesis of hard carbon from gulmohar pod biowaste, wherein the process does not involve any pre / post- treatment and also, covers lesser temperature of pyrolysis around 1000 °C which is economically significant and suitable for bulk and industry level production.

[0065] In another aspect, the present disclosure provides a process for preparing a bio-waste derived hard carbon (HC), comprising the steps of: a) processing the bio-waste to remove impurities; and b) pyrolizing the processed bio-waste under an inert atmosphere followed by powdering to obtain the bio-waste derived hard carbon (HC).

[0066] In various embodiments, the bio-waste comprises pods waste of royal poinciana (Gulmohar pods).

[0067] In certain embodiments, the processing at step a) comprises washing of gulmohar pods with ethanol and deionised water to remove impurities, removing moisture by drying the washed gulmohar pods in an oven at 80- 100 °C and cutting the dried gulmohar pods into uniform pieces. In various embodiments, the pyrolizing at step b) is carried out at 1000 °C for a duration of 3 hours with a heating rate of 5 °C / min.

[0068] In certain embodiments, the inert atmosphere is Ar atmosphere.

[0069] In an embodiment of the present disclosure, the pyrolysis is effected at 900 °C to 1100 °C. For example, 900 °C, 950 °C, 1000 °C, 1050 °C, or 1100 °C. Preferably, 1000 °C.

[0070] In an embodiment of the present disclosure, the pyrolysis is effected for 3-5 hours with a 5 °C.min1ramp rate.

[0071] In an embodiment of the present disclosure, the inert atmosphere is an argon, helium or nitrogen atmosphere; or vacuum.

[0072] In an embodiment of the present disclosure, the gulmohar pod-derived HC has specific surface area (121 m2g-1) and a porous volume (0.33 cm3g-1).

[0073] In yet another aspect, the present disclosure provides a process for preparing an electrode using the bio-waste derived hard carbon (HC), comprising the steps of: a. mixing the bio-waste derived hard carbon (HC), a conducting additive, and a binder into a solvent to obtain a homogenous slurry; and b. depositing the homogenous slurry on a conducting foil configured to act as a current collector followed by drying overnight to obtain the electrode.

[0074] In various embodiments, the bio-waste comprises pods waste of royal poinciana (Gulmohar pods).

[0075] In various embodiments, the bio-waste derived hard carbon (HC), the conducting additive, and the binder are present in a ratio ranging from 60: 10:5 to 80:30:15. In one embodiment, the biowaste derived hard carbon (HC), the conducting additive, and the binder are present in a ratio of 70:20: 10.

[0076] In certain embodiments, the drying at step b. is carried out at a temperature ranging from 60 to 80 °C. In an embodiment of the present disclosure, the binder is selected from but not limited to sodium carboxymethyl cellulose, sodium polyacrylic acid, sodium alginate, polyvinylidene fluoride and a combination thereof.

[0077] In an embodiment of the present disclosure, the conducting additive is selected from but not limited to super P, KJ300, activated carbon, and a combination thereof.

[0078] In an embodiment of the present disclosure, the solvent is N-Methylpyrrolidone (NMP).

[0079] In an embodiment of the present disclosure, the conducting foils is made of aluminium.

[0080] In yet another aspect, the present disclosure provides an energy storage device, comprising the bio-waste derived hard carbon (HC) as a working electrode.

[0081] In another embodiment, the present disclosure provides an energy storage device comprising an electrode coated with gulmohar pod-derived HC coating.

[0082] In another embodiment, the present disclosure provides a sodium-ion energy storage device comprising an electrode coated with gulmohar pod-derived HC coating.

[0083] In another embodiment, the present disclosure provides a sodium-ion energy storage device comprising an anode coated with gulmohar pod-derived HC coating.

[0084] In another embodiment, the present disclosure provides a sodium-ion coin cell (CR2032) comprising an anode coated with gulmohar pod-derived HC coating.

[0085] In an exemplary embodiment, referring to figure 4, the present disclosure provides a sodium-ion coin cell (CR2032) including negative and positive casings (1), sodium metal (2), separator wet with electrolyte (3), working electrode including gulmohar pod-derived HC coating (4), spacer (5) and spring (6).

[0086] In an embodiment of the present disclosure, the electrolyte is selected from the group consisting of but not limited to 1 M NaPF6in EC:DMC, 1 M NaC104in EC:DMC, 1 M NaPF6in PC, 1 M NaPFr, in diglyme.

[0087] In an embodiment of the present disclosure, the separator is Quartz microfiber. In an embodiment of the present disclosure, the sodium-ion coin cell (CR2032) exhibits specific capacity values of 238, 217, 198, 145, 103, 49, and 103 mAh g1at 0.025, 0.05, 0.1, 0.25, 0.5, 1 and 0.5 A g1current density values, respectively.

[0088] In yet another embodiment, the present disclosure is not limited to sodium ion energy storage devices. The gulmohar pod-derived HC disclosed herein, in various forms including activated carbon and carbon nanosheets, may be utilized in a variety of energy-storage devices, such as, a combined battery- supercapacitor energy storage device (also called supercapattery or batpacitor), and an ion energy storage device, as an anode (e.g., in a half-cell), a cathode (e.g., in a half-cell) or both (in a full-cell) with sodium, lithium, and / or any number of other electrolytes and active ions.

[0089] In yet another embodiment, the gulmohar pod-derived HC disclosed herein may be used as electrodes, such as, for instance, an anode, a cathode, as any other supporting material (i.e., secondary addition), etc., for use, for instance, with a variety of energy storage applications, such as, battery, supercapacitor, capacitor, hybrid ion device, and the like.

[0090] EXAMPLES

[0091] The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention.

[0092] Materials:

[0093] The flat strapped pods waste without seeds from royal poinciana trees was used as a carbon precursor. Ethanol and DI water were used for cleaning the precursor. NaPFe, polyvinylidene fluoride (PVDF), and conducting carbon (C65) were procured from Global Nanotech and used as electrolytes. Diglyme, Na metal, and N-methyl-2-pyrrolidone (NMP) from Sigma. Quartz microfiber from Global Nanotech was used as the separator.

[0094] Example 1:- Hard carbon synthesis:

[0095] The waste flat-strapped pods without seeds from royal poinciana trees were cleaned with water, and the upper black layer on the pods was removed and cut down into small pieces. Further, the pieces were washed with DI water and ethanol, followed by drying. The dried pieces were pyrolyzed in Ar atmosphere at 1000 °C for 3 h with a heating rate of 5 °C / min. The obtained material was ground into fine powder.

[0096] The prepared hard carbon from pyrolysis of royal poinciana pods was subjected to various structural characterization techniques for phase purity and confirmation. Figure la shows XRD spectra indicating the presence of two broad peaks at 24.4 and 43.4° representing (002) and (100) planes of hard carbon. The d-spacing calculated for (002) plane was -0.36 nm, which is larger than graphite (0.34 nm) and beneficial for efficient Na+storage. A smaller impurity peak at 26° value is observed, contributing to silicon oxide impurity in royal poinciana pods. The specific surface area of the synthesized hard carbon was determined using the Brunauer-Emmett-Teller (BET) method, yielding a value of 121 m2g1. Figure lb depicts the N2 adsorption-desorption curve, demonstrating a type-III isotherm. Additionally, Figure 1c illustrates micropores as revealed by the NL-DFT pore size distribution. The FESEM images shown in Figures Id- If, represent chunk-like morphology. Nevertheless, its surface presents a rough texture with the appearance of particle-like entities.

[0097] XPS analysis was performed to understand prepared hard carbon's elemental composition and chemical state. The XPS spectra shown in Figure 2a, indicated the presence of C, N, O, and Si. The deconvoluted peaks for C Is, N Is, O Is, and Si 2p are represented in Figure 2b-e. The prepared hard carbon contains an N content of around 1.6 %, with graphitic and pyrrolic N. The SiOi impurity observed in XRD can be seen in XPS Si 2p (Figure 2e).

[0098] Example 2:- Electrode preparation

[0099] The electrodes were fabricated by blending the active material, conducting additive (C65), and PVDF binder in a ratio of 70:20: 10, respectively, utilizing NMP solvent. The resulting slurry was then coated onto a C-coated Al foil, serving as the current collector, and subsequently dried at 80 °C in an oven overnight. Circular electrodes were then fashioned using an electrode cutter with a diameter of 14 mm.

[0100] Example 3:- CR2032 coin cell fabrication:

[0101] The cells were manufactured within an Argon-filled glove box, ensuring an oxygen level below 0.1 ppm and H2O level below 0.1 ppm. The CR2032 cell assembly was employed, with Na serving as the counter and reference electrode. The electrolyte utilized was 1 M NaPFe in diglyme. Quartz microfiber was used as a separator to isolate the negative and positive electrodes.

[0102] Example 4:- Electrochemical testing:

[0103] The galvanostatic charge-discharge measurements on the prepared material were performed using a Neware battery analyzer. The electrochemical impedance spectroscopy analysis (EIS) in the frequency range of 5 mHz - 1000 kHz with an AC amplitude of 10 mV and cyclic voltammetry (CV) was carried out in a Biologic VMP3 multichannel electrochemical workstation equipped with EC Lab software.

[0104] SIB testing for prepared hard carbon was performed in CR2023 coin cell assembly using metallic sodium as counter and reference electrode. Figure 3 shows the electrochemical performance of hard carbon in the 0.01 to 2.7 V potential range. The cyclic voltammetry (CV) shown in Figure 3a represents two broad peaks at 0.78 V and 0.32 V, contributing to the formation of the solid electrolyte interface (SEI) layer caused by electrolyte decomposition at the surface. The reversible sharp peaks of reduction / oxidation at ~ 0.03 / ~ 0.11 V represent the Na+sodiation / desodiation in a graphitic-like domain. The long-term cycling stability of hard carbon was performed at 100 mA g1current density and showed 104 mAh g1specific capacity after 200 cycles. The corresponding charge-discharge curves for specific cycles at 100 mA g1current density are shown in Figure 3c, which displays typical charge-discharge profiles of hard carbon involving sloping region above 0.1 V and plateau region below 0. 1 V correlated with the CV profiles. The sloping region represents Na+intercalation, and the flat plateau region indicates Na+entering the nanopores, called the pore-filling mechanism. Further, rate performance was carried out by varying the current density values. The specific capacity values obtained were 238, 217, 198, 145, 103, 49, and 103 mAh g’1at 0.025, 0.05, 0.1, 0.25, 0.5, 1 and 0.5 A g’1current density values respectively. The stable capacity values were observed at different current density values. Further, the hard carbon was subjected to stability at 25 mA g1current density, which shows 198 mAh g1capacity after 90 cycles. The charge discharge curves for initial some cycles at 25 mA g-1current density is given in Figure 3f.

[0105] Example 5: Sodium ion full cell testing and analysis:

[0106] The sodium-ion full cell was assembled with NasVzCL (NVP) as cathode and hard carbon as anode. The cell was manufactured within an argon-filled glove box, ensuring an oxygen level below 0.1 ppm and H2O level below 0. 1 ppm. The electrochemical performance of full cell was evaluated by assembling 2032 -type coin cell in the voltage range of 2- 3.9V (vs. Na+ / Na). The electrolyte used for full cell testing was IM NaPFe in EC: DEC: 5% FEC. Quartz microfiber was used as a separator.

[0107] Cycling stability study of full cell was conducted at 100 mA g1for 100 cycles. To examine the application feasibility of hard carbon in SIBs (sodium ion batteries), the sodium-ion full cells were assembled with NasVzCU (NVP) cathode and hard carbon anode. Figure 5 shows the cycling data of full cell at 100 mA g '. Full cell cycling data of hard carbon in SIB demonstrates its promising performance. Full cell exhibits a reversible capacity of 85 mAh g1, with a capacity retention up to 64% over 100 cycles of its initial capacity. These results reveals the potential of hard carbon as an efficient anode material for SIB application.

[0108] ADVANTAGES OF THE INVENTION

[0109] • The present disclosure provides a hard carbon material derived from gulmohar pods, wherein spongy structure of the pods are found to be beneficial for HC prepared with N and O doping, higher d-spacing with good electronic conductivity.

[0110] • The present disclosure provides a hard carbon material which avoids acid / alkali pre- or post-treatments and also, covers lesser temperature of pyrolysis around 1000 °C which is economically significant considering the bulk and industry level production.

[0111] • The present disclosure provides a straightforward one-step synthesis route without any pre / post-treatment of hard carbon. The acid washing step, when conducted after the carbonization process, introduces functional groups onto the hard carbon surface. These functional groups play a role in the formation of a weak solid-state interface (SEI) layer on the anode surface during initial cycles.

[0112] • The present disclosure provides a a cost-effective synthesis approach, characterized by simplicity and sustainability.

Claims

WE CLAIM:

1. A hard carbon as electrode for energy storage device, comprising: nitrogen and oxygen doping and a chunk like morphology with interlayer spacing; wherein the hard carbon is a bio-waste derived hard carbon, and has a surface area ranging from 115 to 130 m2g1.

2. A process for preparing the hard carbon as claimed in claim 1, comprising the steps of: a) processing a bio-waste to remove impurities; and b) pyrolizing the processed bio-waste under an inert atmosphere followed by powdering to obtain the hard carbon.

3. The process as claimed in claim 2, wherein the bio-waste comprises pods of waste royal poinciana.

4. The process as claimed in claim 2, wherein the processing at step a) comprises washing of the bio-waste with a solvent mixture comprising of ethanol and water to remove impurities, removing moisture by drying the washed bio-waste in an oven at temperature in the range of 80-100 °C, and cutting the dried bio-waste into uniform pieces.

5. The process as claimed in claim 2, wherein the pyrolizing at step b) is carried out at temperature in the range of 900-1100 °C for a time period in the range of 2-4 hours with a heating rate of 5 °C / min.; and wherein the inert atmosphere is composed of using argon gas or nitrogen gas.

6. A process for preparing an electrode using the hard carbon as claimed in claim 1, comprising the steps of: a. mixing the hard carbon, a conducting additive, and a binder into a solvent to obtain a homogenous slurry; and b. depositing the homogenous slurry on a conducting foil configured to act as a current collector followed by drying for time period in the range of 8-12 h to obtain the electrode.

7. The process as claimed in claim 6, wherein the bio-waste comprises pods waste of Royal poinciana', and wherein the drying at step b) is carried out at a temperature ranging from 60 to 80 °C.

8. The process as claimed in claim 6, wherein the hard carbon, conducting additive, and binder are present in a ratio ranging from 60: 10:5 to 80:30: 15.

9. An energy storage device comprising the hard carbon as claimed in claim 1 as a working electrode.

10. The energy storage device as claimed in claim 9, wherein the energy storage device is selected from a sodium-ion battery, a sodium metal battery, a lithium-ion battery, a lithium metal battery, and lithium-sulfur battery.

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