A method of forming a biomass-derived hard carbon material

Abstract

A method of forming a biomass-derived hard carbon material There is provided a method of forming a biomass-derived hard carbon material comprising reducing the particle size of a biomass material in the presence of an aqueous solvent to form purified biomass material and heat treating the purified biomass material to form biomass-derived hard carbon material. There is also provided an electrode comprising the biomass-derived hard carbon material.

Description

[0001] A method of forming a biomass-derived hard carbon material

[0002] Technical Field

[0003] The present invention relates to a method of forming a biomass-derived carbon material, and to a biomass-derived hard carbon material.

[0004] Background

[0005] Compared to the scarce resources of lithium-ion batteries (LIB), sodium ion batteries (SIB) have gradually become an ideal carrier for large-scale energy storage systems due to their abundant raw material resources. Despite the similarities, the electroactive materials used for LIBs like graphite is not an ideal anode material for SIBs due to the unfavourable thermodynamics of the sodium-graphite intercalation compound, leading to a low sodium storage in graphite. Unlike graphite, hard carbon (HC) materials, a type of non-graphitizable carbon, has been determined to have better sodium storage capabilities due to its unique structure that composed of a mixture of both graphitic and disordered carbon nanodomains.

[0006] Biomass materials have become an ideal hard carbon precursor due to their natural and renewable advantages. However, the huge diversity of biomass resources equates to a large variety of biomass species and the resultant HC material would differ in structure and chemical composition, resulting in varied sodium storage properties. In addition, biomass contains inorganic compounds within is structure, and when converted to HC, the inorganic impurities lead to a poor electrochemical performance in the SIB.

[0007] There is therefore a need for an improved method of forming HC material from biomass.

[0008] Summary of the invention

[0009] The present invention seeks to address these problems, and / or to provide an improved method of forming a biomass-derived hard carbon (HC) material.

[0010] According to a first aspect, there is provided a method of forming a biomass-derived hard carbon material, the method comprising:

[0011] reducing particle size of a biomass material in the presence of an aqueous solvent at room temperature to form purified biomass material; and heat treating the purified biomass material at a temperature of 1000-1600°C to form biomass-derived hard carbon material.The biomass material may be any suitable material. According to a particular aspect, the biomass material may comprise biomass waste material.

[0012] The reducing particle size may be by any suitable method. According to a particular aspect, the reducing particle size may comprise, but is not limited to, ball-milling the biomass material. In particular, the reducing particle size may comprise reducing the particle size of the biomass material to an average particle size of < 20 pm.

[0013] According to a particular aspect, the reducing particle size may be in the presence of any suitable aqueous solvent. For example, the aqueous solvent may comprise, but is not limited to: an organic acid, an inorganic acid, an inorganic base, or a mixture thereof. The aqueous solvent may have a suitable concentration. For example, the concentration may be < 8 M.

[0014] The reducing particle size may be carried out for a suitable period of time. For example, the reducing particle size may be carried out for a period of 30 min-2 hours.

[0015] According to a particular aspect, the heat treating may be carried out under suitable conditions. For example, the heat treating may be carried out in an inert atmosphere. The inert atmosphere may comprise, but is not limited to, argon, nitrogen, or a mixture thereof.

[0016] The heat treating may comprise any suitable heat treating method. For example, the heat treating may comprise, but is not limited to, pyrolysis, Joule heating, or a combination thereof.

[0017] The method may further comprise filtering the purified biomass material to obtain filtrate following the reducing particle size. According to a particular aspect, the method may further comprise drying the filtrate following the filtering.

[0018] The biomass-derived hard carbon material formed from the method may have less impurities as compared to the biomass material. According to a particular aspect, the biomass-derived hard carbon material may have < 15wt. % impurities based on the total mass of the biomass-derived hard carbon material.

[0019] The biomass-derived hard carbon material formed from the method may have a D50 of < 10 pm.According to a particular aspect, the biomass-derived hard carbon may be for use as a negative electrode in a sodium-ion battery.

[0020] According to a second aspect, there is provided an electrode comprising the biomass-derived hard carbon material formed from the method of the first aspect. The electrode may be a negative electrode for a battery, supercapacitor or electrolyzer.

[0021] According to a third aspect, there is provided a battery comprising a positive electrode and a negative electrode, wherein the negative electrode comprises a biomass-derived hard carbon material formed from the method according to the first aspect. The battery may be any suitable battery.

[0022] In particular, the negative electrode may have an initial coulombic efficiency of > 80%. According to another particular aspect, the negative electrode may have a specific discharge capacity of > 350 mAh / g.

[0023] Brief Description of the Drawinqs

[0024] In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

[0025] Figure 1 shows the X-ray diffraction (XRD) spectra of biomass material according to various embodiments;

[0026] Figure 2 shows the XRD spectra of commercial hard carbons according to various embodiments;

[0027] Figure 3 shows the 1stcycle specific charge and discharge capacity versus voltage of biomass materials according to various embodiments;

[0028] Figure 4 shows the rate capability and long-term cycling performance of various biomass and commercial hard carbon materials (1st 2 cycles: 0.02 A g1, next 5 cycles: 0.05 A g-1, next 5 cycles: 0.1 A g-1, 0.2 A g-1, 0.5 A g-1, then 0.1 A g-1for remaining cycles);

[0029] Figure 5 shows the elemental analyses of biomass material lignin treated under various conditions;Figure 6 shows the scanning electron microscope (SEM) of commercial hard carbons and biomass material lignin after treatment under different conditions;

[0030] Figure 7 shows the 1stcycle specific charge and discharge capacity versus voltage of lignin treated under different conditions;

[0031] Figure 8 shows the rate capability and long-term cycling performance of various lignin materials treated under different conditions (1st 2 cycles: 0.02 A g1, next 5 cycles: 0.05 A g1, next 5 cycles: 0.1 A g1, 0.2 A g1, 0.5 A g1, then 0.1 A g-1for remaining cycles);

[0032] Figure 9 shows the 1stcycle specific charge and discharge capacity versus voltage of (a) rubberwood (rubber), (b) pinewood, (c) coconut and (d) woodchip materials treated under different conditions;

[0033] Figure 10 shows the rate capability and long-term cycling performance of (a) rubberwood (rubber), (b) pinewood, (c) coconut and (d) woodchip materials treated under different conditions (1st 2 cycles: 0.02 A g1, next 5 cycles: 0.05 A g1, next 5 cycles: 0.1 A g1, 0.2 A g1, 0.5 A g1, then 0.1 A g-1for remaining cycles); and

[0034] Figure 11 shows the scanning electron microscope (SEM) images of various biomass-derived hard carbon material: rubberwood (rubber), pinewood, coconut and woodchip materials.

[0035] Detailed Description

[0036] As explained above, there is a need for an improved method for forming hard carbon (HC) from biomass.

[0037] In general terms, the present invention provides an improved method for forming HC from biomass, such that the HC may be suitable for use in various applications such as, but not limited to, in battery as an electrode. The biomass-derived HC material may be used as an electrode in sodium-ion batteries. The method is a low-cost method and can be easily scaled up. In particular, the method involves the simultaneous particle sizing and leaching of impurities from the biomass material, followed by heat treatment at a suitable temperature. The heat treatment enables the conversion of the biomass material into HC material. The method results in the formation of a HC material which has improved electrochemical performance.According to a first aspect, there is provided a method of forming a biomass-derived hard carbon material, the method comprising:

[0038] reducing particle size of a biomass material in the presence of an aqueous solvent at room temperature to form purified biomass material; and heat treating the purified biomass material at a temperature of 1000-1600°C to form biomass-derived hard carbon material.

[0039] The biomass material may be any suitable material. For the purposes of the present application, the term “biomass material” refers to a material which is renewable and degradable derived from biological sources through physical, chemical, pyrolysis or biotechnological processes. The biological source may comprise, but is not limited to, plants, animals, and microorganisms. According to a particular aspect, the biomass material may comprise biomass waste material. In particular, the biomass material may comprise, but is not limited to, biochar, lignin, rubberwood, pinewood, coconut, straw, fruit peel, wood waste, bagasse, bamboo, plant stems and leaves, or a mixture thereof.

[0040] The reducing particle size may be by any suitable method. According to a particular aspect, the reducing particle size may comprise, but is not limited to, ball-milling, wet attrition milling, or a combination thereof, the biomass material. In particular, the reducing particle size may comprise reducing the particle size of the biomass material to an average particle size of < 20 pm. For example, the reducing particle size may comprise reducing the particle size to 5-20 pm, 6-18 pm, 6.5-15 pm, 7-12 pm, 8-10 pm, 9-9.5 pm.

[0041] According to a particular aspect, the reducing particle size may be in the presence of any suitable aqueous solvent. The aqueous solvent may comprise, but is not limited to: an organic acid, an inorganic acid, an inorganic base, or a mixture thereof. For example, the aqueous solvent may comprise, but is not limited to, acetic acid, potassium hydroxide, hydrochloric acid, sodium hydroxide, nitric acid, formic acid, sulfuric acid, or a combination or mixture thereof. In particular, the aqueous solvent may comprise acetic acid.

[0042] The aqueous solvent may have a suitable concentration. For example, the concentration may be < 8 M. In particular, the concentration may be 0-8 M, 0.5-7.5 M, 1 -7 M, 2-6 M, 3-5 M, 3.5-4 M.The reducing particle size may be carried out for a suitable period of time. For example, the reducing particle size may be carried out for a period of 30 min-2 hours. In particular, the reducing particle size may be carried out for a period of 30 min-2 hours, 45-100 min, 50-90 min, 60-75 min, 65-70 min. According to a particular aspect, the reducing particle size may be carried out for a period of 30 min.

[0043] The method may further comprise filtering the purified biomass material to obtain filtrate and residue following the reducing particle size. The filtering may be by any suitable manner. The residue may be discarded as it may comprise impurities, while the filtrate may comprise the purified biomass material. The filtrate may then be subjected to the heat treating.

[0044] According to a particular aspect, the method may further comprise drying the filtrate following the filtering and prior to the heat treating. The drying may be by any suitable method. For example, the drying may be by, but not limited to, oven drying, freeze-drying, vacuum drying, microwave drying, or a combination thereof.

[0045] The heat treating may be carried out under suitable conditions. For example, the heat treating may be carried out in an inert atmosphere. The inert atmosphere may comprise, but is not limited to, argon, nitrogen, or a mixture thereof. In particular, the heat treating may be carried out in argon.

[0046] The heat treating may comprise any suitable heat treating method. For example, the heat treating may comprise, but is not limited to, pyrolysis, Joule heating, microwave heating, or a combination thereof.

[0047] The heat treating may be carried out at a suitable temperature. In particular, the heat treating may be carried out at a temperature of 1000-1600°C. For example, the temperature may be 1000-1500°C, 1100-1400°C, 1200-1300°C, 1250-1275°C.

[0048] The heat treating may comprise heating at a heating ramp. The heating ramp may be any suitable temperature ramp applied during the heat treating. The heating ramp may ensure uniform heating and better reproducibility. In particular, the heating ramp may depend on the heat treating method used. For example, the heating ramp may be 5-250°C / min, 25-200°C / min, 50-150°C / min, 100-125°C / min. Alternatively, the heating ramp may be 5-20°C / min, 7-15°C / min, 8-12°C / min, 10-11°C / min, particularly when the heat treating is carried out in a tube or box furnace. In particular, the heating ramp maybe about 10°C / min. Alternatively, the heating ramp may be > 100°C / min when the heat treating is by joule heating or microwave heating. In particular, the Joule heating or microwave heating may enable a rapid heating ramp by directly energizing carbon and biomass materials, thereby producing uniform volumetric heating.

[0049] The heat treating may be carried out for a pre-determined period of time. The predetermined period of time may be any suitable period of time. The pre-determined period of time may be dependent on the biomass material and / or on the heat treating method used. For example, the pre-determined period of time may be > 5 minutes. In particular, the pre-determined period of time may be 5-600 min, 10-540 min, 15-480 min, 30-420 min, 60-360 min, 90-300 min, 120-240 min, 150-180 min. According to a particular aspect, the pre-determined period of time may be > 5 minutes, for example when the heat treating is by Joule heating or microwave heating. According to a particular aspect, the pre-determined period of time may be > 1 hour, for example when the heat treating is carried out in a tube or box furnace. In particular, the pre-determined period of time may be 1 -6 hours, 1 .5-5.5 hours, 2-5 hours, 3-4 hours. Even more in particular, the predetermined period of time may be 1 -3 hours, preferably about 2 hours. The heat treating enables the carbon structures within the purified biomass material to be reorganised.

[0050] Advantageously, the biomass-derived hard carbon material formed from the method may have less impurities and be of a more homogeneous particle size as compared to the biomass material. In this way, when the biomass-derived hard carbon material is used in various applications, improved electrochemical performance may be observed in view of the reduction in the impurities comprised in the hard carbon material and the improved homogeneity in particle size. The improved electrochemical performance may be in terms of improved specific capacity, coulombic efficiency and / or rate capability. According to a particular aspect, the biomass-derived hard carbon material may have < 15 wt. % impurities based on the total mass of the biomass-derived hard carbon material. In particular, the biomass-derived hard carbon material may have 1-15 wt. %, 2-12 wt. %, 5-10 wt. %, 6-9 wt. %, 7-8 wt. % impurities based on the total mass of the biomass-derived hard carbon material.

[0051] The biomass-derived hard carbon material formed from the method may have a D50 of < 10 pm. In particular, the biomass-derived hard carbon material may have a D50 of < 7.5 pm. According to a particular aspect, the biomass-derived hard carbon material may have a D50 of 5-7.5 pm, 5.5-7 pm, 6-6.5 pm.The biomass-derived hard carbon material formed from the method may be used for any suitable application. For example, the biomass-derived hard carbon material may be for use as an electrode. According to a particular aspect, the electrode may be a negative electrode. The electrode may be used as an electrode in a battery, supercapacitor, and the like. The battery may be any suitable battery. For example, the battery may be, but not limited to, sodium-based battery, lithium-based battery, potassium-based battery, magnesium-based battery. In particular, the battery may be, but not limited to, sodium-ion battery, lithium-ion battery, lithium-sulfur battery, a potassium-ion battery, sodium solid state battery, lithium solid state battery, sodium-sulfur battery, magnesium-ion battery, magnesium sulfur battery. Even more in particular, the biomass-derived hard carbon formed may be for use as a negative electrode in a sodium-ion battery.

[0052] As can be seen, the method provides a simple and scalable method for forming biomass-derived hard carbon material. In particular, the method does not involve treatment of biomass material under harsh conditions such as impregnation or soaking in high weight percentage of corrosive solid potassium hydroxide, concentrated phosphoric acid, sulfuric acid, hydrogen fluoride, or costly and cumbersome processes such as hydrothermal, microwave activation, unlike prior art methods. The nature of the solvents used in the method also make the method a green method.

[0053] According to a second aspect, there is provided an electrode comprising the biomass-derived hard carbon material formed from the method of the first aspect. The electrode may be a negative electrode. The electrode may be an electrode for a battery, supercapacitor or electrolyzer. The battery may be any suitable battery, as recited above in relation to the first aspect.

[0054] According to a particular aspect, the electrode may have an initial coulombic efficiency of > 80%. In particular, the electrode may have an initial coulombic efficiency of 80-90%, 82-88%, 83-87%, 84-85%. Even more in particular, the initial coulombic efficiency may be 80-84%.

[0055] According to a particular aspect, the electrode may have a specific discharge capacity of > 350 mAh / g. In particular, the specific discharge capacity may be 350-400 mAh / g, 352-390 mAh / g, 355-380 mAh / g, 358-375 mAh / g, 360-370 mAh / g, 363-365 mAh / g. Even more in particular, the specific discharge capacity may be 350-354 mAh / g.According to a third aspect, there is provided a battery comprising a positive electrode and a negative electrode, wherein the negative electrode comprises a biomass-derived hard carbon material formed from the method according to the first aspect.

[0056] The battery may be any suitable battery. For example, the battery may be but not limited to, sodium-based battery, lithium-based battery, potassium-based battery, magnesium-based battery. In particular, the battery may be, but not limited to, sodium-ion battery, lithium-ion battery, lithium-sulfur battery, a potassium-ion battery, sodium solid state battery, lithium solid state battery, sodium-sulfur battery, magnesium-ion battery, magnesium sulfur battery.

[0057] In particular, the negative electrode may have an initial coulombic efficiency of > 80%. For example, the negative electrode may have an initial coulombic efficiency of 80-90%, 82-88%, 83-87%, 84-85%. Even more in particular, the initial coulombic efficiency may be 80-84%.

[0058] According to another particular aspect, the negative electrode may have a specific discharge capacity of > 350 mAh / g. In particular, the specific discharge capacity may be 350-400 mAh / g, 352-390 mAh / g, 355-380 mAh / g, 358-375 mAh / g, 360-370 mAh / g, 363-365 mAh / g. Even more in particular, the specific discharge capacity may be 350-354 mAh / g.

[0059] Having now generally described the invention, the same will be more readily understood through reference to the following example which is provided by way of illustration, and is not intended to be limiting.

[0060] Example 1

[0061] Materials and method

[0062] Lignin-alkaline (Lignin) was purchased from Tokyo Chemical Industry Co., Ltd (TCI) and pinewood was purchased from Shopee Pte Ltd. Biochar samples (heat treated at ~1000eC) of woodchip, rubberwood, straw, coconut were obtained from Green Energy Investment Holding Private Limited, etc. Commercial HCs - cHC1 (Kuraray Type2 Hard Carbon) and cHC2 (NASICO Hard Carbon), were purchased from ANR Pte. Ltd.To reproduce the conditions of biochar, 8 g of lignin and 7 g of pinewood was heat treated at 1000eC to obtain 1.6 g and 1.5 g of pyrolyzed lignin (Lignin-1000C) and pyrolyzed pinewood (Pinewood-1000C), respectively.

[0063] Synthesis of biomass-derived hard carbon material

[0064] All materials were synthesized in flowing steps: 1 g carbon material was balled milled with 4 mL 4 M Acetic acid for 30 minutes followed by pyrolysis. The pyrolysis was carried out in a high temperature tube furnace (Carbolite Gero) at temperature of 1000-1500°C under dwelling time for 2 hours, using a heating ramp of 10°C / min.

[0065] To compare the efficiency of the ball mill method, a control set up was also used, in which 1 g carbon material was stirred into in 4 mL 4 M Acetic acid for 30 min using magnetic stirrer. The final obtained materials were named based on pyrolysis temperature and treated condition. For example, Lignin-1300C stands for lignin-derived carbon without any chemical treatment (i.e. plain stirring in acetic acid) and heated at 1300°C; Lignin-1300C-washed refers to the sample which was ball-milled with acetic acid and heated to 1300°C, and Lignin-1300C-control represents the sample obtained from heating lignin sample at 1300°C.

[0066] Material characterisation

[0067] The morphology of the samples was investigated by scanning electron microscopy (JEOL JSM7600F). The chemical compositions and crystal structures of samples were conducted using a Bruker D8 Advance X-Ray Diffraction. Elemental analysis of the samples was carried out by X-ray Fluorescence (XRF) spectrometer (Epsilon 1 , Malvern Panalytical). Quantitation of carbon, hydrogen, nitrogen and sulfur (CHNS) was performed by Thermo Scientific Flash Smart Elemental Analyser. Particle size analysis of obtained samples was used by HORIBA Partica LA-960V Laser Scattering Particle Size Distribution Analyzer.

[0068] Preparation of electrode and cell assembly

[0069] be enhanced up to 3 order magnitudes, therefore skin penetration can be well controlled.

[0070] The hard carbon anode materials were mixed with Super P (Alfa Aesar, 99+%) and polyvinylidene difluoride (PVDF) (Solef 5130 PVDF) in a 94:3:3 mass ratio and uniformly dispersed in N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) using mixer (THINKY ARE-250). The obtained slurry was then coated onto an Al foil using the doctor blade casting method and dried at 80°C overnight. The electrode was cut into 10 mm and transferred to glovebox for use. The average loading for the prepared electrode for half-cell was 6.4-7.6 mg cm2. All cell assembling were conducted in an Ar-filled MBRAUN glove box with H2O and O2content below 0.1 ppm. The half-cell was assembled using 2032-coin cells with Na foils as counter electrode and glass fiber (Whatman) as the separator. 1 M NaPF6in diethylene glycol dimethyl ether (diglyme, Sigma-Aldrich) was used as an electrolyte. The galvanostatic charging and discharge cycling was conducted on a LAND battery testing system within a voltage range of 0.005 V to 2.0 V vs Na / Na+under different current densities: 0.02 A g1, 0.05 A g1, 0.1 A g1, 0.2 A g1, 0.5 A g1.

[0071] Results

[0072] XRF and CHNS analysis on Lignin-1000C, Pinewood-1000C and biochar samples revealed varied levels and types of impurities, and carbon content, as seen in Table 1 .

[0073]

[0074] *Determined from CHNS analysis; ** Major impurities: Na, S, K, Ca; and Other impurities: Si, P, Cl, Mn, Fe, Cu, etc.

[0075] Table 1: Elemental analysis of various biomass and commercial hard carbon material

[0076] The impurity levels for the biomass samples were much higher and carbon content was lower for the biomass samples as compared to commercial HCs (cHC1 and cHC2). The presence of these impurities, or the lack thereof, was manifested in their corresponding XRD spectra - biochar samples of woodchip, straw and rubberwood showed unknownXRD signals, as can be seen in Figure 1 . A broad XRD signal at -24° for amorphous carbon was observed for all samples, including the commercial HCs, as can be seen in Figures 1 and 2. Notably, the commercial HCs possessed an additional XRD signal at 27°, corresponding to graphite (JCPDS 00-056-0159), as can be seen in Figure 2.

[0077] The electrochemical properties of the materials were evaluated in a half cell with sodium metal as the counter electrode. All materials revealed a steep, sloping curve above ~0.1 V, representing adsorption-interlayer intercalation and followed by a plateau below ~0.1 V, representing interlayer intercalation / pore filling process, as seen in Figure 3. Here, the first cycle charge curves were shorter than the discharge curves, indicating coulombic losses from non-faradic reactions such as formation of solid-electrolyte interphase. As shown in Figure 4, the initial coulombic efficiencies (CE) of lignin-1000c and straw were below 50%, while the CE of the rest of the biochar samples ranged between 60-80%. Only cHC1 and cHC2 had initial CE values higher than 80%. The reversible capacities at their corresponding current densities varies followed the trend of: cHC1 > cHC2 > pinewood-1000C > coconut > rubberwood « woodchip > straw > lignin-1000C. Thus, it can be seen that the concentration of impurities in the biomass material negatively correlates with the electrochemical performance of HC.

[0078] Using lignin-1000C as the representative biomass material, it was then subjected to the treatment process as follows. Lignin-1000C was ball-milled in aqueous acetic acid, a component of household vinegar, followed by filtration to wash away the excess acetic acid. The filtrate was dried in an oven and subsequently subjected to heat treatment at 1300°C to obtain lignin-1300C (washed).

[0079] A control experiment was carried out where lignin-1000C was soaked in acetic acid without ball mill and heat treated at 1300°C, to obtain a material annotated as lignin-1300C-controL Other combinations of acid / base were also attempted and were found to be effective in decreasing the impurity contents, as can be seen in Figure 5.

[0080] The effect of ball milling resulted in a homogenous particle sizing similar to that of commercial HCs, as can be seen in Table 2. SEM revealed that lignin-1300C(washed) has a morphology similar to that of cHC1 and significantly smaller than lignin-1000C, lignin-1300C and lignin-1300C (control), as can be seen in Figure 6.Std.

[0081] Samples D50 (pm) Mean (pm) Dev.

[0082] (pm) cHCl 7.2 20.3 36.2 cHC2 9.2 11.9 20.9 Lignin-1300C(washed) 6 6.3 2.5 Lignin-13OOC 48.7 51.9 22.9 Pinewood-1300C (washed) 5.6 5.9 2.0 Pinewood-1300C 268.3 486.5 515.6 Rubber-1300C(washed) 6.5 8.8 15 Rubber-13OOC 19.5 33.6 37.3 Coconut-1300C (washed) 6.3 10.1 21.5 Coconut-1300C 43.7 49.9 39.5 Woodchip-1300C(washed) 7.1 7.7 3.5 Woodchip-1300C 331.3 534.7 538.4

[0083] Table 2: Particle size analysis of various biomass and commercial hard carbon material

[0084] Battery performance was evaluated and compared against all the lignin-based materials, as shown in Figures 7 and 8. It was determined that the initial CE and specific discharge capacity of lignin-1300C-washed, lignin-1300C-control, lignin-1300C and lignin-1000C were 80.7% and 350 mAh g1, 70.5% and 352 mAh g1, 55.6% and 292 mAh g1, 32.9% and 325 mAh g1, respectively.

[0085] This showed that the treatment process was highly effective, resulting in doubling of CE as compared to untreated lignin-1000C and, greater than 10% increase in CE and / or greater than 16% increase in capacity as compared to lignin-1300C-control and lignin-1300C.

[0086] The treatment process was also applied to other biomass samples like rubberwood, pinewood, coconut and woodchip, and the corresponding battery performance all showed improvement when the treatment process was applied as compared to the control samples (see Figure 9). XRF results showed a decrease in impurities aftertreatment on all the biomass materials, as seen in Table 3. Particle sizes were also smaller and more homogeneous after treatment, and became comparable with commercial HCs, cHC1 and cHC2 (Table 2).

[0087] Other impurities Major Impurity (wt%)

[0088] Samples Treatment Conditions (wt%)

[0089] Na, S, K, Ca

[0090] Si, P, Cl, Mn, Fe, Cu Lignin Nil 12.529 (9.258 (5), 0.415 (K), 2.856 (Na and Ca)) 0.483 Lignin Heat treatment @ 1300°C 9.86 (8.533 (S), 0.653 (K), 0.674(Na and Ca)) 0.449 Lignin Wash and heat treatment @ 13OO°C 5.612 ( 4.644(5), 0.215 (K), 0.753(Na and Ca)) 0.516 Rubber Nil 17.491(0.12(5), 5.819(K), 11.55(Ca)) 1.131 Rubber Heat treatment @ 1300°C 19.06 (0.125(5), 6.153(K), 12.79(Ca)) 1.232 Rubber Wash and heat treatment @ 1300°C 5.686 (0.169(5), 2.419(K), 3.098 (Ca)) 1.089 Coconut Nil 6.667(0.064(5), 5.806(K), 0.797(Ca)) 1.004 Coconut Heat treatment @ 1300°C 4.61(0.14(5), 4.091(K) , 0.379(Ca)) 0.567 Coconut Wash and heat treatment @ 1300°C 5.461 (0.06(5), 5.006(K), 0.395(Ca) 0.448 Pinewood Nil 8.513 (0.050(5), 1.249(K), 7.214(Ca)) 1.415 Pinewood Heat treatment @ 1300°C 11.799 (0.049(5), 1.515(K), 10.235(Ca)) 1.048 Pinewood Wash and heat treatment @ 1300°C 6.379 (0.120(5), 1.829(K), 4.43(Ca)) 1.073 Woodchip Nil 14.413 (0.088(5), 3.152 (K), 11.173 (Ca)) 0.694 Woodchip Heat treatment @ 1300°C 21.833 (0.131(s), 4.152(K), 17.55(Ca)) 0.859 Woodchip Wash and heat treatment @ 1300°C 4.812 (0.186(s), 1.516(K), 3.11(Ca)) 1.08 cHCl NIL 2.628 (0.031(s), 2(K), 0.597 (Ca)) 0.939 cHC2 NIL 0.341(0.26(5), 0.012(K), 0.069 (Ca) 1.018

[0091] Table 3: Elemental analysis of various biomass and commercial hard carbon material

[0092] The treatment was particularly effective for rubberwood, where post-treated rubberwood, i.e. rubberwood-1300C-washed, resulted in an initial CE and specific discharge capacityof 82.3% and 352 mAh g1, respectively, much higher than that of untreated rubberwood-1300C (70.0% and 346 mAh g1) (Figure 9a).

[0093] For pinewood, treatment (pinewood-1300C-washed) resulted in an initial CE and specific discharge capacity of 84.2% and 357 mAh g1, respectively, as compared to 84.6% and 351 mAh g1for the untreated material (pinewood-1300C, Figure 9b). Noteworthily, the specific discharge capacity at a higher rate of 0.5 A g1was 179 mAh g1for pinewood-1300C-washed, -10% higher than pinewood-1300C which was at 155 mAh g-1(Figure 10b).

[0094] For coconut, the initial CE and specific discharge capacity of coconut-1300C-washed and coconut-1300C were 87.3% and 359 mAh g1and 83.3% and 353 mAh g1, respectively (Figure 9c).

[0095] For woodchip, the initial CE and specific discharge capacity of woodchip-1300C-washed and woodchip-1300C were 86.7% and 382 mAh g1and 83.9% and 346 mAh g1, respectively (Figure 9d).

[0096] All the biomass-derived hard carbon materials exhibited excellent electrochemical reversibility, with almost no loss in specific capacity over 100 cycles (Figure 10 a-d). Further, all the biomass-derived hard carbon materials formed had a similar morphology to cHC1 , as can be seen in Figure 11.

[0097] Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.

Claims

Claims1. A method of forming a biomass-derived hard carbon material, the method comprising:- reducing particle size of a biomass material in the presence of an aqueous solvent at room temperature to form purified biomass material; and - heat treating the purified biomass material at a temperature of 1000- 1600°C to form biomass-derived hard carbon material.

2. The method according to claim 1, wherein the reducing particle size comprises ball-milling the biomass material.

3. The method according to claim 1 or 2, wherein the reducing particle size comprises reducing the particle size of the biomass material to an average particle size of < 20 pm.

4. The method according to any preceding claim, wherein the aqueous solvent comprises: an organic acid, an inorganic acid, an inorganic base, or a mixture thereof.

5. The method according to any preceding claim, wherein the aqueous solvent has a concentration of < 8 M.

6. The method according to any preceding claim, wherein the reducing the particle size is carried out for a period of 30 min - 2 hours.

7. The method according to any preceding claim, wherein the heat treating is carried out in an inert atmosphere.

8. The method according to claim 7, wherein the inert atmosphere comprises argon, nitrogen, or a mixture thereof.

9. The method according to any preceding claim, wherein the heat treating comprises pyrolysis, Joule heating, or a combination thereof.

10. The method according to any preceding claim, wherein the biomass-derived hard carbon material has < 15wt. % impurities based on the total mass of the biomass-derived hard carbon material.

11. The method according to any preceding claim, wherein the biomass-derived hard carbon material has a D50 of < 10 pm.

12. The method according to any preceding claim, wherein the method further comprises filtering the purified biomass material to obtain the filtrate following the reducing particle size.

13. The method according to claim 12, wherein the method further comprises drying the filtrate following the filtering.

14. The method according to any preceding claim, wherein the biomass material comprises biomass waste material.

15. The method according to any preceding claim, wherein the biomass-derived hard carbon material is for use as a negative electrode in a sodium-ion battery.

16. An electrode comprising the biomass-derived hard carbon material formed from the method according to any preceding claim.

17. The electrode according to claim 16, wherein the electrode is a negative electrode for a battery, supercapacitor or electrolyzer.

18. A battery comprising a positive electrode and a negative electrode, wherein the negative electrode comprises a biomass-derived hard carbon material formed from the method according to any of claims 1 -15.

19. The battery according to claim 18 or the electrode according to claim 17, wherein the battery is a sodium-ion battery, a lithium-ion battery, a lithium-sulfur battery, magnesium-ion battery, or a potassium-ion battery.

20. The battery according to claim 18 or the electrode according to claim 17, wherein the negative electrode has an initial coulombic efficiency of > 80%.

21. The battery according to claim 18 or the electrode according to claim 17, wherein the negative electrode has a specific discharge capacity of > 350 mAh / g.

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