PROCESS FOR PREPARING AND USE OF HARD-CARBON CONTAINING MATERIALS

Abstract

The invention relates to a process for preparing hard carbon-containing material with a specific surface area of 100 m.sup.2/g or less, comprising the utilisation of one or more animal-derived materials.

Claims

1. A process for preparing a hard carbon-containing material comprising the utilisation of one or more animal faeces-derived materials.

2. The process for preparing a hard carbon-containing material according to claim 1, comprising the steps: a) providing a composition comprising one or more animal faeces-derived materials; b) where the composition in step a) comprises one or more uncharred animal faeces-derived materials, heating the composition at a temperature of from 150° C. to 700° C. to char the one or more animal faeces-derived materials to produce a charred animal faeces-derived materials; c) treating the composition comprising charred animal faeces-derived materials from either step a) or step b) to remove any unwanted metal-ion- and/or non-metal-ion-containing impurities; and d) pyrolysing the treated charred animal faeces-derived materials from step c) at a temperature of greater than 700° C. to 2500° C.

3. The process according to claim 2 wherein the composition comprising one or more animal faeces-derived materials provided in step a) comprises less than 10% by weight of mineral-containing impurities.

4. The process according to claim 2 where treatment step c) comprises a chemical digestion using alkaline and/or acid conditions.

5. The process according to claim 1, wherein the hard carbon-containing material exhibits a BET (N.sub.2) specific surface area of 100 m.sup.2/g or less.

6. The process according to claim 6, wherein the hard carbon-containing material has a carbon-content of at least 90% by weight.

7. The process according to claim 6, wherein the hard carbon-containing material comprises up to 50% by weight of metal-ion- and/or non-metal-ion-containing components.

8. The process according to claim 1, wherein the one or more animal faeces-derived materials comprise human faeces-derived materials.

9. (canceled)

10. A hard carbon-containing material made by the process according to claim 1.

11. A hard carbon-containing material made by the process according to claim 2.

12. An electrode comprising the hard carbon-containing material according to claim 11.

13. An energy storage device comprising one or more electrodes according to claim 12.

14. A hard carbon-containing material derived from one or more animal faeces-derived materials having a BET (N.sub.2) specific surface area of 100 m.sup.2/g or less.

15. An electrode comprising the hard carbon-containing material according to claim 14.

16. An energy storage device comprising one or more electrodes according to claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0067] The present invention will now be described with reference to the following figures in which:

[0068] FIG. 1 shows a flow chart to illustrate a preferred process of the present invention;

[0069] FIG. 2 shows an XRD pattern for the hard carbon-containing material derived from chicken manure using the process of the present invention described in Example 1;

[0070] FIG. 3 depicts sodiation/desodiation voltage (vs. sodium) profiles for the hard carbon material described in Example 1 which underwent a rate capability test from C/20 to 1C.

[0071] FIG. 4 depicts sodiation/desodiation voltage (vs. sodium) profiles for the hard carbon material described in Example 1 which underwent a rate capability test from C/50 to C/10.

[0072] FIG. 5 illustrates the cycle-life performance of an animal-derived hard-carbon-containing anode active material made according to Example 1 in a full sodium-ion cell formed between 1.0 and 4.2 V for two cycles followed by long-term cycling between 1.0 and 4.0 V at a charge and discharge rate of C/5 with a CV step of C/50.

[0073] FIG. 6 illustrates a voltage-capacity curve obtained from a three-electrode cell using anode material made according to Example 1.

[0074] FIG. 7 shows an XRD pattern for acid-washed sewage sludge prior to de-mineralisation in io Example 2; and

[0075] FIG. 8 shows an XRD pattern for carbon material extracted from de-mineralised sewage sludge before pyrolysis in Example 2.

[0076] FIG. 9A shows a sodiation/desodiation voltage (versus sodium) profile obtained for a comparative hard carbon material with a high specific surface area (in the range 500-1000 m.sup.2/g), prepared according to Comparative Example 3 (method as described in CN 107887602A).

[0077] FIG. 9B shows a sodiation/desodiation (versus sodium) profile for a hard carbon material prepared according to the process of the present invention.

[0078] FIG. 10A shows an electron micrograph of hard carbon-containing material prepared according to Comparative Example 3, reproduced from CN107887602A.

[0079] FIG. 10B shows an electron micrograph of hard carbon-containing material prepared according to the present invention which demonstrates that the material has no visible micropores.

DETAILED DESCRIPTION

[0080] Hard carbon materials were made according to the pre process of the present invention as detailed below in Examples 1 and 2. Comparative Example 3 follows a method like that disclosed in prior art document CN107887602A.

EXAMPLE 1

Hard Carbon-Containing Material Prepared According to the Process of the Present Invention Using Chicken Manure-Derived Material

[0081] Obtained pelleted chicken manure is milled down to <1 mm and dispersed in water at a volume ratio of 1:6. The aqueous dispersion is agitated by stirring on a stirring plate and the inorganic impurities (e.g. primary silicates) were at least partially separated from the mixture by sedimentation due to the density of the rock-forming inorganic compounds commonly found in the chicken manure being higher than either the water or the biomass (heavy media separation). The biomass-rich supernatant is then extracted in a separate container by the means of reduced pressure to yield powdered chicken manure with reduced inorganic impurities.

[0082] The powdered reduced inorganic impurity chicken manure with is then rinsed with an organic solvent, e.g. acetone, and dried at 100° C. overnight. The organic solvent is used to accelerate the drying and reduce the foul smell, but is not essential. The inorganic impurity content of the dried powder is estimated by calcining a portion of (ca. 100 mg) of the dried powder at 1000° C. under atmospheric air and weighing the residual ash. The ash content of the dry powder is 9.9 wt. % which is significantly less than the 26.9 wt. % initial ash content of the as-received chicken manure. The powdered reduced inorganic impurity chicken manure is then charred at 600° C. for 4 hr under 1 L/min flow of argon, at a yield of ca. 40 wt. %. Higher charring temperatures are avoided to minimise the crystallisation of the remaining silica. The obtained charred (carbonised) chicken manure is then treated under reflux in boiling 4.0 M NaOH solution for 6 hr in air to minimise the impurity content. After rinsing the powder with boiling deionised water, chemical digestion is continued using boiling 2.0 M HCl solution for 6 hr to further minimise the content of calcium, potassium, phosphorus and magnesium compounds as well as common transition metals and their oxides such as iron and manganese. The resulting powder is rinsed again in boiling deionised water, dried and pyrolysed at 1200° C. for 3 hr at a ramp rate of 2° C./min under 1 L/min flow of argon with a 1-hour dwell at 1000° C., at a yield of ca. 77 wt. %. The powder is then milled to d.sub.50 of ca. 10 μm and filtered through a 25 pm sieve to exclude larger particles.

[0083] Product Analysis using XRD

[0084] Analysis by X-ray diffraction techniques is conducted using a Siemens® D5000 powder diffractometer to confirm that the desired target materials had been prepared, to establish the phase purity of the product material and to determine the types of impurities present. From this information it is possible to determine the lattice parameters of the unit cells.

[0085] The general XRD operating conditions used to analyse the materials are as follows:

[0086] Slits sizes: 2 mm, 2 mm, 0.2 mm

[0087] Range: 2θ=10°-60°

[0088] X-ray Wavelength=1.5418 Å (Angstroms) (Cu Ka)

[0089] Speed: 1.0 seconds/step

[0090] Increment: 0.025°

[0091] Table 1 below provides details of an estimation of the spacing of the graphitic crystallites as io well as their domain size (in-plane: La and stacking: Lc), using the information from the XRD pattern shown in FIG. 2.

TABLE-US-00001 TABLE 1 (002) 2-θ position Spacing XRD Lc XRD La Process ° nm Nm nm 1200° C. (acid and 23.93 0.371 2.134 6.716 alkali digestions)

EXAMPLE 2

Hard Carbon-Containing Material Prepared According to the Process of the Present Invention Using Human-Derived Waste Material (Sewage Sludge)

[0092] In a typical recycling process, wet sewage sludge is dewatered, dried and carbonised to obtain a biochar rich in phosphorous and minerals which can be directly used as phosphorous-rich fertilizer. To obtain a suitable biochar precursor for hard carbon synthesis, further treatment is required. De-mineralisation and de-phosphorisation of sewage sludge biochar is carried out in a molten alkali bath. During this process, dried sewage sludge biochar is mixed in equal weight proportion with NaOH powder in a glass container. The mixture is then heated at 500° C. for 3 hours in an oven under atmospheric air. The product collected after molten alkali digestion is rinsed multiple times with deionised water to remove the digested metal-containing impurities and all the residual NaOH. An x-ray diffraction pattern of the purified carbon obtained after the process of de-mineralisation is presented in FIG. 8. From this purified sewage sludge biochar, hard carbon is obtained through a pyrolysis process under inert atmosphere. Typically, the purified sewage sludge biochar is heated up to 1300° C. with a ramp rate of 2° C./min with an intermediate dwell time of 1 h at 1000° C. and is kept at 1300° C. for 3 hours, before being cooled down to room temperature for collection.

[0093] Electrochemical Results

[0094] Anodes comprising hard carbon-containing materials made according to Examples 1 and 2 are prepared by solvent-casting a slurry comprising the hard carbon material derived from pyrolysed animal-derived material (as described above), binder and solvent, in a weight ratio 92:6:2. A conductive carbon such as C65™ carbon (Timcal)® may be included in the slurry. PVdF is a suitable binder, and N-Methyl-2-pyrrolidone (NMP) may be employed as the solvent. The slurry is then cast onto a current collector foil (e.g. aluminium foil) and heated until most of the solvent evaporates and an electrode film is formed. The anode electrode is then dried further under dynamic vacuum at about 120° C.

[0095] Cell Testing

[0096] For half-cell tests, Hard Carbon electrodes are paired with one disk of sodium metal as reference and counter electrode. Glass Fibre GF/A is used as the separator and a suitable electrolyte is also used. Any suitable Na-ion electrolyte may be used, preferably this may comprise one or more salts, for example NaPF6, NaAsF6, NaClO4, NaBF4, NaSCN and Na triflate, in combination with one or more organic solvents, for example, EC, PC, DEC, DMC, EMC, glymes, esters, acetates etc. Further additives such as vinylene carbonate and fluoro ethylene carbonate may also be incorporated. A preferred electrolyte composition comprises 0.5 M NaPF6!EC:PC:DEC.

[0097] All cells were rested for 24 h prior to cycling. For three-electrode tests, Hard Carbon is used as negative electrode, a standard oxide material is used as positive electrode and a piece of sodium is used as reference, all three electrodes are wet by the same electrolyte. As separator, two polyethylene membranes of 24.5 urn thickness were used.

[0098] The half-cells are tested using Constant Current cycling technique, and the three electrode cells are tested using Constant Current—Constant Voltage technique.

[0099] The cell is cycled at a given current density between pre-set voltage limits. A commercial battery cycler from MTI Inc. (Richmond, Calif., USA) was used. On charge, alkali ions are inserted into the hard carbon-containing anode material. During discharge, alkali ions are extracted from the anode and re-inserted into the cathode active material.

[0100] The cell cycling results are shown as a function of anode specific capacity (rather than cathode specific capacity) as this is more informative for this application. The anode specific capacity is calculated by dividing the measured capacity by the mass of active component in the anode.

[0101] Results: Electrochemical Testing of the Hard Carbon Material According to Example 1

[0102] Specific capacity of the resulting hard carbon was obtained from half-cells. The results are io summarised in Table 2 below.

TABLE-US-00002 TABLE 2 Reversable sodiation/desodiation specific capacity of hard carbon derived from chicken manure biochar at different rates vs. sodium metal. Reversible sodiation/desodiation specific capacity vs. sodium [mAh/g] Rate Cell 1 Cell 2 Cell 3 Cell 4 C/50 — — 300/291 287/279 C/20 236/224 254/241 — — C/10 172/169 180/174 180/174 197/190 C/5 116/114 130/126 — — C/2 74/72 79/76 — — 1 C 39/38 42/41 — —

[0103] FIG. 3 and FIG. 4 depict representative sodiation/desodiation voltage (vs. sodium) profiles, numerical values are also reported in Table 2. The equivalent current rate used at 1C is 190 mA/g. Cell 2 (FIG. 3) is representative of the rate capability between C/20 and 1C (sodiation at C/20, desodiation at increasing current rate). Cell 4 (FIG. 4) is representative of the rate capability between C/50 and C/10 (sodiation/desodiation at C/50 followed by sodiation at C/10 and desodiation at C/20). From the voltage profiles, especially at low current rate, two section can be identified. The first section being a sloping region, attributed to alkali ions storage within layers and/or defects, and the second section being a plateau region, attributed to alkali ions storage within pores and voids. The voltage profiles and rate capabilities of animal-derived Hard Carbon (in particular, from chicken-manure biochar) prove to be in line with previously reported Hard Carbon materials.

[0104] FIG. 5 illustrates the cycle-life performance of a representative animal-derived hard-carbon-containing anode active material in a full sodium-ion cell formed between 1.0 and 4.2 V for two cycles at Constant Current of C/10 with Constant Voltage step until C/100, followed by long-term cycling between 1.0 and 4.0 V at a charge and discharge rate of C/5 with a Constant Voltage step until C/50. 79% of the discharge capacity was retained after 130 cycles at current rate of C/5, proving that good reversible alkali ion storage can be attained within the animal-derived hard-carbon.

[0105] FIG. 6 illustrates representative voltage-capacity curves obtained from a three-electrode cell. The capacity is provided based on active mass of hard carbon. The sodium-ion full cell performances prove the suitability of animal-derived waste biochar material as a precursor for synthesis of hard carbon anode material.

EXAMPLE 3

Comparative Hard Carbon Material with a High Specific Surface Area (Above 580 m.SUP.2./g)

[0106] A biaochar sample was mixed with sodium hydroxide and heated up to 650° C. This was followed by a neutralisation step which resulted in a hard carbon-containing material with a high specific surface area of 500-1000 m.sup.2/g.

[0107] As shown in FIG. 9A, the resulting hard carbon material exhibits sodiation and desodiation behaviour of this material with a large irreversible capacity; the first cycle coulombic efficiency is only 23.3%.

[0108] By contrast, the first cycle coulombic efficiency for hard carbon material produced according to the present invention (i.e. with a much lower surface area), as shown in FIG. 9B, is 80.9%. FCCE is of particular importance in the case of secondary batteries with fixed inventory of charge carriers.

[0109] It is believed that the poor first cycle coulombic efficiency results are due to the produced material having an excessively high surface area (500-1000 m.sup.2/g) resulting from the high temperature used in the alkali treatment step and ii) the lack of a secondary high-temperature (>700° C.) heat treatment (referred to as ‘pyrolysis’ in the process of the present invention) which results in oxygen and nitrogen groups from the alkaline/acid digestion being retained on the surface of the hard carbon-containing material. Electron micrographs of the hard carbon-containing materials made according to Comparative Example 3 and according to the present invention are shown in FIGS. 10A and 10B, respectively. As can be seen, the FIG. 10A illustrates a highly porous hard carbon-containing material, whereas FIG. 10B illustrates an essentially non-porous hard carbon-containing material.

EXAMPLE 4

Experiments to Demonstrate the Need for the Charring, Chemical Treatment and Pyrolysis Steps in the Process of the Present Invention

[0110] A key advantage provided by the process of the present invention is to maximise the amount and/or quality (including purity) of the hard carbon material which can be produced from animal-derived material. The way this is achieved is by using an initial charred material which is treated to selectively remove various impurities before it undergoes pyrolysis—i.e. all three steps; charring, treating to remove impurities and pyrolysis, are required, and they also need to be performed in this specific order. This is demonstrated by the iterations detailed in Table 3 below:

TABLE-US-00003 TABLE 3 Ash Pyrolysis content yield Processing steps (wt. %) (wt. %) Comments 1 Ground .fwdarw. Dried >27 <10 Low pyrol- ysis yield Low purity 2 Ground .fwdarw. Separated from >10 <10 Low pyrol- sand .fwdarw. Dried ysis yield Low purity 3 Ground .fwdarw. Separated from <2 >60 High pyrol- sand .fwdarw. Carbonised at 500° ysis yield C. .fwdarw. Leached in pure boiling High purity NaOH .fwdarw. Digested in boiling acid solution .fwdarw. Rinsed .fwdarw. Dried 4 Ground .fwdarw. Separated from <2 <5 Low charring sand .fwdarw. Digested in cold yield acid solution .fwdarw. Rinsed .fwdarw. High purity Dried 5 Ground .fwdarw. Separated from <1 <5 Very low sand .fwdarw. Leached in boiling charring yield NaOH solution .fwdarw. Digested in High purity boiling acid solution .fwdarw. Rinsed .fwdarw. Dried

[0111] As Table 3 shows, chemical treatment steps (acid-digestion, leaching in NaOH, etc.) will be effective to purify (remove inorganic/metal/non-metal impurities) an animal-derived starting material, however, carrying out such chemical treatment steps on non-carbonised (uncharred) animal-derived material will significantly solubilise the organic components of the animal-derived starting material and will result in a poor post-pyrolysis yield. Additionally, it is favourable to perform the chemical treatment steps before pyrolysis to remove the oxygen groups from the surface of the final hard carbon-containing material (as discussed above). Further, an initial carbonisation is essential to lock the carbon atoms in a carbonous matrix (rather than the initial inorganic compounds) so that the carbon content is preserved in following chemical treatments. Iteration 3 is found to provide the highest pyrolysis yield as well as an acceptable purity level.