Reduced graphene oxide-based aerogels and electrodes

Abstract

The present invention relates to reduced graphene oxide (rGO)-based aerogel and methods of preparing such aerogels The invention also relates to electrode materials and electrodes comprising such aerogels and alkali-ion batteries comprising such electrodes.

Description

Reduced graphene oxide-based aerogels and electrodesField of the Invention

[0001] The present invention relates to a reduced graphene oxide (rGO)-based aerogel and methods of preparing such aerogels. The invention also relates to electrode materials and electrodes comprising such aerogels, and alkali-ion batteries comprising such electrodes. However, it will be appreciated that the invention is not limited to these particular fields of use.Background of the Invention

[0002] The following discussion of the prior art is provided to place the invention in an appropriate technical context and enable the advantages of it to be more fully understood. It should be appreciated, however, that any discussion of the prior art throughout the specification should not be considered as an express or implied admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0003] Graphene-based materials have shown great potential in alkali-ion batteries (for example, Li-ion batteries (LIBs)) both as active materials and as a conductive matrix to accommodate other electroactive species. The atomic-thin layer of carbon atoms packed by covalent carbon bonds with a continuous conjugated network gives graphene high mechanical strength and flexibility to accommodate stress generated during battery operations, remarkable electrical conductivity for fast electron transport, and a large surface area for facile ion transport across the electrode-electrolyte interface. When stacked, few-layer graphene offers interlayer channels for ion intercalation. Therefore, combining graphene with commonly used electrode materials, for example, transition metal oxides (TMOs), has been considered an effective strategy for producing high-performance electrodes in alkali-ion batteries.

[0004] TMOs have been explored as promising electrode materials (for example, anode materials) in high-performance alkali-ion batteries because of their higher theoretical capacity (for example, 500-1000 mAh / g) than that of commercial graphite electrodes (for example, 372 mAh / g). Such high-capacity values of TMOs originate from the electrochemical conversion reaction that leads to complete conversion of the TMO into the transition metal and Li2O. However, TMOs based anodes, for example iron oxide-based anodes, suffer from several limitations, including poor electrical conductivity, structural instability due to the large volume change caused by alkali-ion (for example, Li+) insertion and extraction, and the tendency of the particles to aggregate over continuous cycling, all of which lead to limited cycling performance of TMOs in practical applications.

[0005] In order to resolve these issues, many efforts have been directed at distributing TMO nanoparticles (for example, iron oxide nanoparticles) in various forms of carbon-basedsystems. A typical method to prepare iron oxide / rGO begins with the synthesis of Fe-precursor- graphene oxide (GO) composites, followed by the annealing process. This synthesis route has been commonly used but is complicated and requires the usage of various chemicals, such as urea and sodium acetate. To simplify the process, a hydrothermal route has been proposed, which sidesteps the preparation of precursor composites and typically involves ammonia to avoid the formation of byproducts. In the hydrothermal method, the reduction of GO and the formation of iron oxides occur simultaneously in a liquid medium. It is known that rGO sheets tend to aggregate in aqueous media during the reducing process, which may result in inhomogeneous dispersion of iron oxides nanoparticles. Moreover, the re-stacking of rGO laminates occurs during the charging / discharging process. Such collapse in the exfoliated structure of graphene induces a loss in active sites and impedes the ion diffusion process. Consequently, it is challenging to reach the expected capacity for various forms of rGO-based electrodes.

[0006] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0007] It is an object of at least one preferred form of the present invention to provide improved transition metal oxide aerogels or transition metal aerogels for electrode materials. It is an object of further preferred forms of the present invention to simultaneously provide rGO (from GO) and to form the transition metal oxide or transition metal (from a salt) and in the form of an aerogel. It is an object of yet further preferred forms of the present invention to provide a preparation method which provide a durable electrode structure that inhibits graphene restacking during electrode cycling, and transition metal oxides or transition metal from aggregation.Summary of the Invention

[0008] According to a first aspect, the present invention provides a method of preparing an rGO-based aerogel, the method comprising: a. providing a dispersion comprising GO and a transition metal salt; b. lyophilising the dispersion to obtain a transition metal salt / GO aerogel; and c. heating the transition metal salt / GO aerogel in an inert atmosphere to obtain a transition metal oxide / rGO aerogel or a transition metal / rGO aerogel.

[0009] The inventors of the present application have surprisingly found a method of preparing an rGO-based aerogel that may prevent the rGO sheets from restacking and the transition metal oxide or transition metal from aggregation when used as an electrode materialfor batteries during charging-discharging cycles. Without wishing to be bound by theory, the present inventors contemplate such advantages are achieved by separating the steps of preparing an aerogel from a mixture of GO and the transition metal salt, and the thermal treatment of such aerogels. When dispersed (for example, in water), oxygenated functional groups on GO render GO sheets negatively charged, thus attracting transition metal cations to be attached. After being transformed into aerogels via freeze-drying, GO can be thermally reduced by decomposing oxygen-containing groups into gases under an inert atmosphere, and simultaneously, reacting the transition metal salt with oxygen-containing groups on GO, mostly epoxy and hydroxyl groups, leading to the formation of transition metal oxide or transition metal. In the hydrothermal treatment, the most common method of producing transition metal oxide / rGO composites (for example, iron oxide / rGO composites), rGO sheets tend to agglomerate to lower the surface energy and may restack, which could lead to the inhomogeneous arrangement of transition metal oxide nanoparticles on the sheets.

[0010] Advantageously, the present invention may allow the formation of a highly porous rGO network and a relatively uniform distribution of transition metal oxide or transition metal particles through transition metal cations anchoring onto GO sheets, and the reduction of GO. Further, the aerogels may advantageously be directly used as free-standing electrodes without adding binder and conductive agents due to the interconnected high-conductivity network furnished by rGO. Moreover, the present invention may advantageously allow the formation of hierarchically porous rGO matrix with a homogeneous distribution of transition metal oxide or transition metal particles, which can provide interconnected ion diffusion channels and adequate buffer space to resist volume change during cycling. Additionally, the high surface area derived from the rGO layer enables a large contact area between the electrode and electrolyte when used in a battery. By way of example, and in one preferred embodiment as disclosed herein, a highly and hierarchically porous Fe3O4 / rGO electrode exhibits high capacity (1224 mAh g-1at 100 mA g-1) and excellent cycling stability (a retention rate of 82.8% at 1 A g-1after 1000 cycles).

[0011] In some embodiments, the GO is in the form of GO sheets.

[0012] In some embodiments, the rGO is in the form of rGO sheets.

[0013] In some embodiments, the GO and the transition metal salt are uniformed dispersed in the dispersion.

[0014] In some embodiments, the method further comprises a step of uniformly dispersing the GO and the transition metal salt to form a homogenous mixture.

[0015] In some embodiments, the method further comprises a step of sonicating a mixture of the GO and the transition metal salt to uniformly disperse the GO and the transition metal salt.

[0016] In some embodiments, the sonication has a duration of about 0.1 hrs to about 10 hrs, for example, about 0.1 hrs to about 1 hr, about 1 hr to about 2 hrs, about 2 hrs to about 3 hrs, about 3 hrs to about 4 hrs, about 4 hrs to about 5 hrs, about 5 hrs to about 6 hrs, about 6 hrs to about 7 hrs, about 7 hrs to about 8 hrs, about 8 hrs to about 9 hrs, about 9 hrs to about 10 hrs, about 0.1 hrs to about 5 hrs, about 5 hrs to about 10 hrs, about 0.1 hrs to about 3 hrs, about 3 hrs to about 6 hrs, about 0.1 hrs, about 1 hr, about 2 hrs, about 3 hrs, about 4 hrs, about 4 hrs, about 4 hrs, about 5 hrs, about 6 hrs, about 7 hrs, about 8 hrs, about 9 hrs, about 10 hrs.

[0017] In some embodiments, the transition metal is selected from Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Ta, W, Re, Os, Ir, Pt, Au, Hg, or any combinations thereof.

[0018] In some embodiments, the transition metal is selected from Fe, Co, Ni, Cr, or any combinations thereof. In preferred embodiments, the transition metal is Fe.

[0019] In some embodiments, the transition metal oxide is selected from FeO, Fe3O4, Fe2O3, a-Fe2O3, p-Fe2O3, y-Fe2O3, £-Fe2O3, or any combinations thereof.

[0020] In preferred embodiments, the transition metal oxide is Fe3O4.

[0021] In some embodiments, the transition metal is Fe, the transition metal oxide is iron oxide, and the rGO-based aerogel is an iron oxide / rGO aerogel. In certain embodiments, the iron oxide is Fe3O4.

[0022] In some embodiments, the transition metal in the transition metal salt has an ion charge of +l, +II, +III, +IV, +V, +VI , or any combinations thereof.

[0023] In some embodiments, the transition metal is iron, and iron in the iron salt has an ion charge of +II, +III or a combination thereof.

[0024] In some embodiments, the transition metal salt is a halide salt, carbonate, sulphate, acetate, oxalate, or any combinations thereof, preferably a halide salt.

[0025] In preferred embodiments, the transition metal salt is FeCI3.

[0026] In some embodiments, the step c comprises heating the transition metal salt / GO aerogel for about 1 to about 10 hours, for example, about 1 hours to about 2 hours, about 2 hours to about 3 hours, about 3 hours to about 4 hours, about 4 hours to about 5 hours, about5 hours to about 6 hours, about 6 hours to about 7 hours, about 7 hours to about 8 hours, about 8 hours to about 9 hours, about 9 hours to about 10 hours, about 1 hours to about 5 hours, or about 5 hours to 10 hours, or about 1 hours, about 2 hours, about 3 hours, about 4 hours, about 5 hours, about 6 hours, about 7 hours, about 8 hours, about 9 hours, about 10 hours, preferably about 3 hours.

[0027] In some embodiments, a concentration of GO in the dispersion is about 0.1 mg / ml to about 10 mg / ml, for example about 0.1 mg / ml to about 0.2 mg / ml, about 0.2 mg / ml to about 0.3 mg / ml, about 0.3 mg / ml to about 0.4 mg / ml, about 0.4 mg / ml to about 0.5 mg / ml, about 0.5 mg / ml to about 0.6 mg / ml, about 0.6 mg / ml to about 0.7 mg / ml, about 0.7 mg / ml to about 0.8 mg / ml, about 0.8 mg / ml to about 0.9 mg / ml, about 0.9 mg / ml to about 1 mg / ml, about 1 mg / ml to about 1.5 mg / ml, about 1.5 mg / ml to about 2 mg / ml, about 2 mg / ml to about 2.5 mg / ml, about2.5 mg / ml to about 3 mg / ml, about 3 mg / ml to about 3.5 mg / ml, about 3.5 mg / ml to about 4 mg / ml, about 4 mg / ml to about 4.5 mg / ml, about 4.5 mg / ml to about 5 mg / ml, about 5 mg / ml to about 5.5 mg / ml, about 5.5 mg / ml to about 6 mg / ml, about 6 mg / ml to about 6.5 mg / ml, about6.5 mg / ml to about 7 mg / ml, about 7 mg / ml to about 7.5 mg / ml, about 7.5 mg / ml to about 8 mg / ml, about 8 mg / ml to about 8.5 mg / ml, about 8.5 mg / ml to about 9 mg / ml, about 9 mg / ml to about 9.5 mg / ml, about 9.5 mg / ml to about 10 mg / ml, about 0.1 mg / ml to about 2 mg / ml, about 2 mg / ml to about 4 mg / ml, about 4 mg / ml to about 6 mg / ml, about 6 mg / ml to about 8 mg / ml, about 8 mg / ml to about 10 mg / ml, about 0.1 mg / ml to about 5 mg / ml, or about 5 mg / ml to about 10 mg / ml, or about 0.1 mg / ml, about 0.2 mg / ml, about 0.3 mg / ml, about 0.4 mg / ml, about 0.5 mg / ml, about 0.6 mg / ml, about 0.7 mg / ml, about 0.8 mg / ml, about 0.9 mg / ml, about 1 mg / ml, about 1.5 mg / ml, about 2 mg / ml, about 2.5 mg / ml, about 3 mg / ml, about 3.5 mg / ml, about 4 mg / ml, about 4.5 mg / ml, about 5 mg / ml, about 5.5 mg / ml, about 6 mg / ml, about 6.5 mg / ml, about 7 mg / ml, about 7.5 mg / ml, about 8 mg / ml, about 8.5 mg / ml, about 9 mg / ml, about 9.5 mg / ml, about 10 mg / ml. Preferably, the concentration is about 1 mg / ml.

[0028] In some embodiments, a weight ratio of the transition metal salt and the GO in the dispersion is about 10:1 to about 1 :10, for example, about 10:1 to about 9.5:1 , about 9.5:1 to about 9:1 , about 9:1 to about 8.5:1 , about 8.5:1 to about 8:1 , about 8.5:1 to about 8:1 , about 8: 1 to about 7.5:1 , about 7.5: 1 to about 7:1 , about 7: 1 to about 6.5:1 , about 6.5: 1 to about 6: 1 , about 6:1 to about 5.5:1 , about 5.5:1 to about 5:1 , about 5:1 to about 4.5:1 , about 4.5:1 to about 4:1 , about 4:1 to about 3.5:1 , about 3.5:1 to about 3:1 , about 3:1 to about 2.5:1 , about 2.5:1 to about 2:1 , about 2:1 to about 1.5:1 , about 1.5:1 to about 1 :1 , about 1 :1 to about 1.5:1 , about 1.5:1 to about 2:1 , about 2: 1 to about 2.5:1 , about 2.5: 1 to about 3: 1 , about 3: 1 to about 3.5:1 , about 3.5: 1 to about 4: 1 , about 4: 1 to about 4.5: 1 , about 4.5: 1 to about 5: 1 , about 5: 1 to about 5.5:1 , about 5.5: 1 to about 6: 1 , about 6: 1 to about 6.5:1 , about 6.5: 1 to about 7:1 , about 7: 1 to about 7.5:1 , about 7.5: 1 to about 8: 1 , about 8: 1 to about 8.5:1 , about 8.5: 1 to about 9: 1 , about9:1 to about 9.5:1 , about 9.5:1 to about 10:1 , about 10:1 to about 5:1 , about 5:1 to about 1 :1 :, about 1 : 1 to about 5: 1 , about 5: 1 to about 10: 1 , about 10:1 to about 1 : 1 , or about 1 : 1 to about 1 :10, or about 10:1 , about 9.5:1 , about 9:1 , about 8.5:1 , about 8:1 , about 7.5:1 , about 7:1 , about 6.5:1 , about 6:1 , about 5.5:1 , about 5:1 , about 4.5:1 , about 4:1 , about 3.5:1 , about 3:1 , about2.5:1 , about 2: 1 , about 1.5:1 , about 1 :1 , about 1.5:1 , about 2:1 , about 2.5:1 , about 3: 1 , about3.5:1, about 4:1 , about 4.5:1 , about 5:1 , about 5.5:1 , about 6:1 , about 6.5:1 , about 7:1 , about7.5:1 , about 8:1 , about 8.5:1 , about 9:1 , about 9.5:1 , about 10:1.

[0029] In some embodiments, the dispersion is an aqueous dispersion. In some embodiments, the GO and the transition metal salt are dispersed in deionised water.

[0030] In some embodiments, the step b comprises lyophilising the dispersion at a temperature of about -100°C to about -50°C, for example, about -100°C to about -95°C, about -95°C to about -90°C, about -90°C to about -85°C, about -85°C to about -80°C, about -80°C to about -75°C, about -75°C to about -70°C, about -70°C to about -65°C, about -65°C to about - 60°C, about -60°C to about -55°C, about -55°C to about -50°C, about -100°C to about -90°C, about -90°C to about -80°C, about -80°C to about -70°C, about -70°C to about -60°C, about - 60°C to about -50°C, about -100°C, about -90°C, about -80°C, about -70°C, about -60°C, about -50°C, preferably about -60°C.

[0031] In some embodiments, the step c comprises heating the transition metal salt / GO aerogel in an atmosphere of an inert gas comprising N2, Ar, Ne, He, or any combinations thereof.

[0032] In some embodiments, the inert gas has a flow rate of about 1 seem to about 100 seem, for example, about 1 seem to about 5 seem, about 5 seem to about 10 seem, about 10 seem to about 15 seem, about 15 seem to about 20 seem, about 20 seem to about 25 seem, about 25 seem to about 30 seem, about 30 seem to about 35 seem, about 35 seem to about 40 seem, about 40 seem to about 45 seem, about 45 seem to about 50 seem, about 50 seem to about 55 seem, about 55 seem to about 60 seem, about 60 seem to about 65 seem, about 65 seem to about 70 seem, about 70 seem to about 75 seem, about 75 seem to about 80 seem, about 80 seem to about 85 seem, about 85 seem to about 90 seem, about 90 seem to about 95 seem, about 95 seem to about 100 seem, about 1 seem to about 25 seem, about 25 seem to about 50 seem, about 50 seem to about 75 seem, about 75 seem to about 100 seem, about 1 seem to about 10 seem, about 10 seem to about 20 seem, about 20 seem to about 30 seem, about 30 seem to about 40 seem, about 40 seem to about 50 seem, about 50 seem to about 60 seem, about 60 seem to about 70 seem, about 70 seem to about 80 seem, about 80 seem to about 90 seem, about 90 seem to about 100 seem, or about 1 seem, about 5 seem, about 10 seem, about 15 seem, about 20 seem, about 25 seem, about 30 seem, about 35 seem, about40 seem, about 45 seem, about 50 seem, about 55 seem, about 60 seem, about 65 seem, about 70 seem, about 75 seem, about 80 seem, about 85 seem, about 90 seem, about 95 seem, about 100 seem, "seem" stands for standard cubic centimetres per minute.

[0033] In some embodiments, the step c comprises heating the transition metal salt / GO aerogel at a temperature of about 100 °C to about 1000 °C, or about 200 °C to about 600 °C, for example, about 100 °C to about 200 °C, about 200 °C to about 300 °C, about 300 °C to about 400 °C, about 400 °C to about 500 °C, about 500 °C to about 600 °C, about 600 °C to about 700 °C, about 700 °C to about 800 °C, about 800 °C to about 900 °C, about 900 °C to about 1000 °C, about 100 °C to about 300 °C, about 300 °C to about 500 °C, about 500 °C to about 700 °C, about 700 to about 900 °C, about 100 °C to about 500 °C, about 300 °C to about 700 °C, or about 200 °C to 800 °C, or about 400 °C to about 1000 °C, or about 100 °C, about 150 °C, about 200 °C, about 250 °C, about 300 °C, about 350 °C, about 400 °C, about 450 °C, about 500 °C, about 550 °C, about 600 °C, about 650 °C, about 700 °C, about 750 °C, about 800 °C, about 850 °C, about 900 °C, about 950 °C, about 1000 °C.

[0034] In some embodiments, the step c comprises heating the transition metal salt / GO aerogel at a temperature of above about 200 °C, above about 300 °C, or above about 400 °C, for example, above about 200 °C, above about 250 °C, above about 300 °C, above about 350 °C, above about 400 °C, above about 450 °C, above about 500 °C, above about 550 °C, above about 600 °C. Preferably the transition metal salt / GO aerogel is heated for a sufficient time and at a sufficient temperature and under sufficient conditions to thereby obtain a transition metal oxide / rGO aerogel or a transition metal / rGO aerogel.

[0035] In preferred embodiments, the step c comprises heating the transition metal salt / GO aerogel at a temperature of above about 400 °C.

[0036] In some embodiments, wherein the transition metal oxide / rGO aerogel or the transition metal / rGO aerogel has a thickness of about 50 pm to about 500 pm, for example, about 50 pm to about 100 pm, about 100 pm to about 150 pm, about 150 pm to about 200 pm, about 200 pm to about 250 pm, about 250 pm to about 300 pm, about 300 pm to about 350 pm, about 350 pm to about 400 pm, about 400 pm to about 450 pm, about 450 pm to about 500 pm, about 50 pm to about 200 pm, about 200 pm to about 350 pm, about 350 pm to about 500 pm, about 50 pm, about 100 pm, about 150 pm, about 200 pm, about 250 pm, about 300 pm, about 350 pm, about 400 pm, about 450 pm, about 500 pm.

[0037] In some embodiments, the method does not involve the use of any other chemicals.

[0038] According to a second aspect of the present invention there is provided an rGO- based aerogel obtained from the method according to the first aspect of the present invention.

[0039] According to a third aspect of the present invention there is provided an electrode material comprising the rGO-based aerogel according to the second aspect of the present invention.

[0040] According to a fourth aspect of the present invention there is provided an electrode comprising or consisting of the electrode material according to the third aspect of the present invention, or the use of the electrode material according to the third aspect of the present invention in an electrode.

[0041] In some embodiments, the electrode is devoid or substantially devoid of a binder.

[0042] In some embodiments, the electrode is free-standing. The skilled person would understand that a free-standing electrode does not include a current collector.

[0043] The skilled person would understand energy density is one of the most important performance characteristics of a battery (for example, alkali-ion batteries) and it is highly related to the electroactive material loading mass as well as the storage sites (for example, alkali metal storage sites) in the electrode. Current battery electrodes are mostly fabricated by mixing electroactive materials with binders and then coating on current collectors, which include supporting materials that cannot store energy and thus limit their energy density. Advantageously, the electrodes of the present invention may provide free-standing and binder- free features that may minimise the addition of non-electroactive materials and therefore significantly improve the energy density of batteries. Moreover, these electrodes of the present invention may avoid alkali-ion plating (for example, lithium-ion plating) as found on graphite electrodes.

[0044] According to a fifth aspect of the present invention there is provided a method of preparing an electrode, comprising the steps of: a. obtaining the rGO-based aerogel according to the second aspect of the present invention; and b. pressing the rGO-based aerogel into an aerogel film to thereby obtain an electrode.

[0045] In some embodiments, the method further comprises a step of removing any moisture from the aerogel film.

[0046] In some embodiments, the electrode is an anode.

[0047] In some embodiments, the electrode material is an anode material.

[0048] According to a sixth aspect of the present invention there is provided an electrode obtained by the method according to the fifth aspect of the present invention.

[0049] According to a seventh aspect of the present invention there is provided an alkali metal-ion battery comprising the electrode according to the fourth or sixth aspect of the present invention.

[0050] In some embodiments, the battery is a lithium-ion battery, a sodium-ion battery, or a potassium-ion battery.

[0051] In some embodiments, the battery has a specific capacity of about 300 mAh / g to about 2000 mAh / g at a current density of about 0.1 A / g to 5 A / g, for example, about 300 mAh / g to about 350 mAh / g, about 350 mAh / g to about 400 mAh / g, about 400 mAh / g to about 450 mAh / g, about 450 mAh / g to about 500 mAh / g, about 500 mAh / g to about 550 mAh / g, about 550 mAh / g to about 600 mAh / g, about 600 mAh / g to about 650 mAh / g, about 650 mAh / g to about 700 mAh / g, about 700 mAh / g to about 750 mAh / g, about 750 mAh / g to about 800 mAh / g, about 800 mAh / g to about 850 mAh / g, about 850 mAh / g to about 900 mAh / g, about 900 mAh / g to about 950 mAh / g, about 950 mAh / g to about 1000 mAh / g, about 1000 mAh / g to about 1050 mAh / g, about 1050 mAh / g to about 1100 mAh / g, about 1100 mAh / g to about 1150 mAh / g, about 1150 mAh / g to about 1200 mAh / g, about 1200 mAh / g to about 1250 mAh / g, about 1250 mAh / g to about 1300 mAh / g, about 1300 mAh / g to about 1350 mAh / g, about 1350 mAh / g to about 1400 mAh / g, about 1400 mAh / g to about 1450 mAh / g, about 1450 mAh / g to about 1500 mAh / g, about 1500 mAh / g to about 1550 mAh / g, about 1550 mAh / g to about 1600 mAh / g, about 1600 mAh / g to about 1650 mAh / g, about 1650 mAh / g to about 1700 mAh / g, about 1700 mAh / g to about 1750 mAh / g, about 1750 mAh / g to about 1800 mAh / g, about 1800 mAh / g to about 1850 mAh / g, about 1850 mAh / g to about 1900 mAh / g, about 1900 mAh / g to about 1950 mAh / g, about 1950 mAh / g to about 2000 mAh / g, about 300 mAh / g to about 500 mAh / g, about 500 mAh / g to about 700 mAh / g, about 700 mAh / g to about 900 mAh / g, about 900 mAh / g to about 1100 mAh / g, about 1100 mAh / g to about 1300 mAh / g, about 1300 mAh / g to about 1500 mAh / g, about 1500 mAh / g to about 1700 mAh / g, about 1700 mAh / g to about 1900 mAh / g, or about 300 mAh / g, about 400 mAh / g, about 500 mAh / g, about 600 mAh / g, about 700 mAh / g, about 800 mAh / g, about 900 mAh / g, about 1000 mAh / g, about 1100 mAh / g, about 1200 mAh / g, about 1300 mAh / g, about 1400 mAh / g, about 1500 mAh / g, about 1600 mAh / g, about 1700 mAh / g, about 1800 mAh / g, about 1900 mAh / g, about 2000 mAh / g at a current density of about 0.1 A / g to about 0.5 A / g, about 0.5 A / g to about 1 A / g, about 1 A / g to about 1.5 A / g, about 1.5 A / g to about 2 A / g, about 2 A / g to about 2.5 A / g, about 2.5 A / g to about 3 A / g, about 3 A / g to about 3.5 A / g, about 3.5 A / g to about 4 A / g, about 4 A / g to about 4.5 A / g, about 4.5 A / g to about 5 A / g, about 0.1 A / g to about 1 A / g, about 1 A / g to about 2 A / g, about 2 A / g to about 3 A / g, about 3 A / g to about 4 A / g, about 4 A / g to about 5 A / g, about 0.1 A / g, about 0.5 A / g, about 1 A / g, about 1.5 A / g, about 2 A / g, about 2.5 A / g, about 3 A / g, about 3.5 A / g, about 4 A / g, about 4.5 A / g, about 5 A / g.

[0052] In some embodiments, the battery has a specific capacity of above about 400 mAh / g at a current density of about 2A / g.

[0053] In some embodiments, the battery has a specific capacity of above about 600 mAh / g at a current density of about 1A / g for over about 1000 charging-discharging cycles.

[0054] In some embodiments, the battery has a specific capacity of about 1100mAh / g after about 100 charging-discharging cycles.

[0055] In some embodiments, the battery has a capacity retention rate of at least about 60% after about 1000 charging-discharging cycles at a current density of about 1 A / g, for example, at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, or at least about 90%.Definitions

[0056] In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set out below. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one having ordinary skill in the art to which the invention pertains.

[0057] Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”.

[0058] As used herein, the phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” (or variations thereof) appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole. As used herein, the phrase “consisting essentially of” limits the scope of a claim to the specified elements or method steps, plus those that do not materially affect the basis and novel characteristic(s) of the claimed subject matter.

[0059] With respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter may include the use of either of the other two terms. Thus, in some embodiments not otherwise explicitly recited, any instance of “comprising” may be replaced by “consisting of’ or, alternatively, by “consisting essentially of”.

[0060] Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term “about”. The examples are not intended to limit the scope of the invention. In what follows, or where otherwise indicated, “%” will mean “weight %”, “ratio” will mean “weight ratio” and “parts” will mean “weight parts”.

[0061] The term ‘substantially’ as used herein shall mean comprising more than 50% by weight, where relevant, unless otherwise indicated.

[0062] The recitation of a numerical range using endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1 , 1.5, 2, 2.75, 3, 3.80, 4, 5, etc.).

[0063] Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

[0064] The terms “preferred” and “preferably” refer to embodiments of the invention that may afford certain benefits, under certain circumstances. However, other embodiments may also be preferred, under the same or other circumstances. Furthermore, the recitation of one or more preferred embodiments does not imply that other embodiments are not useful and is not intended to exclude other embodiments from the scope of the invention.

[0065] It must also be noted that, as used in the specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise.

[0066] As used herein, with reference to numbers in a range of numerals, the terms “about,” “approximately” and “substantially” are understood to refer to the range of -10% to +10% of the referenced number, preferably -5% to +5% of the referenced number, more preferably -1 % to +1 % of the referenced number, most preferably -0.1 % to +0.1 % of the referenced number. Moreover, with reference to numerical ranges, these terms should be construed as providing support for a claim directed to any number or subset of numbers in that range. For example, a disclosure of from 1 to 10 should be construed as supporting a range of from 1 to 8, from 3 to 7, from 1 to 9, from 3.6 to 4.6, from 3.5 to 9.9, from 8 to 10, and so forth.

[0067] The term “aerogel” refers to a porous material derived from a gel or a gel-like material in which the liquid component(s) is replaced with a gas.

[0068] The term “lyophilising” refers to a process in which water in the form of ice under low pressure is removed from a material by sublimation.

[0069] The term “alkali metal-ion battery” refers to a type of rechargeable battery in which alkali metal ions move from the negative electrode through an electrolyte to the positive electrode during discharge and back when charging.

[0070] The term “electrode material” refers to the material that may be coated on a surface of a current collector to form an electrode. Alternatively, the electrode material itself may be compressed to form an electrode.

[0071] The term “specific capacity” refers to the total amount of electricity generated and / or stored due to electrochemical reactions in a battery in comparison to the weight of the active material and is expressed in ampere or milliampere hours per gram (Ah / g or mAh / g).

[0072] The term “charging-discharging cycle” refers to the full discharge of a charged battery with subsequent recharge, or the full charge of an empty battery with subsequent recharge, at a specific current.

[0073] Although exemplary embodiments of the disclosed technology are explained in detail herein, it is to be understood that other embodiments are contemplated. Accordingly, it is not intended that the disclosed technology be limited in its scope to the details of construction and arrangement of components set forth in the following description or illustrated in the drawings. The disclosed technology is capable of other embodiments and of being practiced or carried out in various ways.Brief Description of the Drawings

[0074] Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0075] Figure 1 shows a schematic diagram of an embodiment of the invention for preparing an rGO-based aerogel.

[0076] Figure 2 shows compositional and morphological characterisation of FesC / rGO composites, (a) Rietve Id- refined fit of the XRD data with the FesO4 structural model for FG2- 400 and FG2-600. (b), (c), and (d) SEM images of FG2-600. (e) C, (f) Fe, and (g) O EDX mapping of FG2-600 corresponding to the SEM image (d).

[0077] Figure 3 shows XRD patterns of aerogels FG2-200, FG2-400, FG2-600, FG 1-400, FG2-400, and FG3-400.

[0078] Figure 4 shows SEM images of aerogel samples and membrane samples, (a) and (b), FG2-400; (c) and (d), FGM-400. (e) Fe and (f) O elemental EDX mapping of FGM-400 corresponding to (d).

[0079] Figure 5 shows synthesis and physicochemical attributes, (a) Fe wt% in all samples confirmed by ICPMS. (b) The electrical conductivity of all samples was measured via a four- probe method, (c) C1s XPS spectra of G-400 and FG2-400. G-400 indicates pure rGO aerogel obtained at 400 °C.

[0080] Figure 6 shows the TGA curves of (a) G-400 and (b) FG2-400.

[0081] Figure 7 shows atomic percentage of C, O, Fe, and Cl in (a) FG1-400, FG2-400, FG3-400, (b) FG2-200, FG2-400, FG2-600 according to XPS analysis. C1s XPS peak of (c) FG2-400, (d) FG3-400, (e) FG2-200, and (f) FG2-600.

[0082] Figure 8 shows electrochemical performance of FesO^rGO composites, (a) CV curves of FG2-600 for the first four cycles at a scan rate of 0.1 mV / s. (b) Discharge-charge profiles of the FG2-600 electrode. The comparison between the cyclic performance of (c) FG1- 400, FG2-400, FG3-400, G-400, FGM-400, and (d) FG2-400, and FG2-600 at a current density of 100 mA / g within the potential window between 0.01 V and 3 V. Cross sectional features of (e) FG2-400 and (f) FG2-600 electrodes, (g) ECSA results for FG2-400 and FG2-600 at a scan rate of 0.03 mV / min. (h) Rate performance of FG2-400 and FG2-600. (i) Cycling stability of FG2-600 at a current density of 0.5 A / g and 1 A / g, respectively.

[0083] Figure 9 shows voltage-capacity curves of (a) G-400, (b) FG 1-400, (c) FG2-200, (d) FG2-400, (e) FG3-400, and (f) FGM-400 in the 1st, 2nd, 25th and 50th cycle when applying a constant current of 100 mA / g from 0.01 V to 3 V.

[0084] Figure 10 shows (a) cycling performance and (b) coulombic efficiency of G-200, G- 400, and G-600. Coulombic efficiency of (c) FG1-400, FG2-400, FG3-400, (d) FG2-200, FG2- 400, and FG3-600. Cycling performance of (e) FG2-200, (f) FG1-200, FG1-400, and FG1-600. These cycling data were obtained at a current rate of 100 mA / g within 0.01 V to 3 V voltage window.

[0085] Figure 11 shows Cross sectional SEM images of (a) and (b) FG1-400, (c) and (d) FG3-400, and (e) and (f) G-400.

[0086] Figure 12 shows postmortem and mechanistic analysis of Fe3O4 / rGO electrodes, (a) Operando XRD of FG2-600 in the first two cycles. The Nyquist plots of (b) FG 1-400, FG2-400, FG3-400, and FGM-400, and (c) FG2-200, FG2-400, and FG2-600. The changes in Nyquist plots of (d) FG2-400 and (e) FG2- 600 after 10 and 50 cycles. SEM images of FG2-600 after 50 cycles from a top view (f) and side view (g), respectively.

[0087] Figure 13 shows changes in XRD patterns in FG2-600 after 10 cycles and 50 cycles, respectively.

[0088] Figure 14 shows the equivalent circuit for EIS analysis.

[0089] Figure 15 shows SEM images of FG2-600 which was electrochemically cycled 50 times, (b) Fe, (c) C, and (g) O EDX mapping of cycled FG2-600 corresponding to the SEM image (a).

[0090] Figure 16 shows comparison between the electrochemical performance of the embodiments of the present invention against FesO^graphene-based LIB anodes reported in the literature.Detailed Description of the Invention

[0091] The skilled addressee will understand that the invention comprises the embodiments and features disclosed herein as well as all combinations and / or permutations of the disclosed embodiments and features.

[0092] The following examples illustrate an embodiment of the invention in which an FesO^rGO aerogel is prepared and used in battery to test its performance. However, the skilled person would appreciate that the examples may apply to other transition metal oxides / rGO aerogels and transition metal / rGO aerogels.Preparation of FesO^rGO aerogels

[0093] GO, synthesized via a modified Hummer’s method, was supplied by NiSiNa Materials Japan. However, the skilled person would appreciate that GO may be obtained from alternative methods or suppliers. The concentration of GO dispersion in deionised water is 1 mg / ml. After adding various amounts of FeCh powder into GO, the mixture was sonicated for 3 h to form a uniformly dispersed suspension and then freeze-dried to obtain aerogel samples. Subsequently, these aerogels were heated in an N2 atmosphere at varied temperatures ranging from 200 °C to 600 °C for the purpose of reducing GO and oxidising FeCh. The final aerogel product is denoted as, for example, FGx-y, where FG represents FesO^rGO, x is the weight ratio of FeCh and GO applied in the preparation process, and y corresponds to the annealing (or heating) temperature. For example, FG1-400 means that the aerogel is obtained with a FeCh and GO weight ratio of 1 , and an annealing temperature of 400 °C. A schematic diagram of the formation of the aerogels is shown in Figure 1.Preparation of FesO^rGO membranes

[0094] For comparison, FesO^rGO membranes were made by filtrating FeCh / GO mixture through a polyvinylidene fluoride (0.22 pm) membrane under vacuum and then peeling off. These composite membranes were subsequently thermally treated at 400 °C under N2, and the resultant membranes are denoted as FGM-400.Characterisation

[0095] The surface morphology and elemental distribution of samples before and after cycling were characterized by field emission scanning electron microscopy (FE-SEM, FEI Nova NanoSEM 450) and energy-dispersive X-ray spectroscopy (EDX, FEI Nova NanoSEM 450 fitted with an SDD-EDS detector), respectively. The thickness of these freestanding electrodes was measured based on the cross-sectional images captured by SEM. The X-ray diffraction technique (XRD, PANalytical Empyrean 1 Cu Source) was used to analyze the phase composition of FG composites (which include the membranes and aerogels). The phase evolution of the electrode during battery cycling was probed by Operando XRD using a homemade cell equipped with a glassy carbon window. FexOyto rGO weight ratio in the composites was confirmed by inductively coupled plasma mass spectrometry (ICPMS, 0ptima7000 and Avio from PerkinElmer, USA), and by thermogravimetric analysis (TGA, PerkinElmer Simultaneous Thermal Analyzer 8000) performed under O2 atmosphere from room temperature (22 °C) to 800 °C with a heating rate of 5 °C / min. The X-ray photoelectron spectroscopy (XPS, Thermo Scientific Al-Ka radiation) was used to investigate the chemical composition of the samples. The electrical conductivity was measured by a four-probe method (Ossila four-point probe).Electrochemical measurements

[0096] LIB half-cells (CR2032 coin cells) were assembled to test the electrochemical performance of the samples as potential anode materials. After mechanically pressing with a hydraulic laminating press (MTI Corporation), FesOVrGO aerogels transformed into freestanding thin films, which were then punched into 12 mm discs and transferred to a vacuum oven to remove residual moisture. These thin films were then directly used as the electrode with Li metal as the counter electrode. 1 M LiPFe in 1 :1 (volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) was employed as the electrolyte. The galvanostatic charge-discharge (GCD) cycling was performed on a Neware BTS battery cycler. Cyclic voltammetry measurements (CV, BioLogic VMP-3e) were conducted from 0.01 V to 3 V at a scan rate of 0.1 mV / s. Electrochemical impedance spectroscopy (EIS, BioLogic VMP-3e) was conducted by applying an alternating current perturbation of 10 mV in 1 MHz-100 mHz frequency range. Prior to the electrochemically active surface area (ECSA) measurement, samples were discharged-charged one time and rested till the equilibrium was established. ECSA measurements were followed at a scan rate of 0.03 mV / min within 10 mV potential window with respect to the open-circuit voltage (OCV).Results - phases and crystal structure

[0097] The phases and crystal structure of the materials were analysed by XRD, as presented in Fig. 2 and Fig. 3. Rietveld analysis was conducted using a structural model of FesC and XRD data. Background, zero offset, lattice and atomic parameters were refined, and the size term was refined in order to estimate the particle size distribution from the measured peak shapes (shown in Table 1).FG2-400, Fd-3m FG2-600, Fd-3m a 8.4252(7) 8.4027(6)A ASize 15 nm 16 nmOxygen positional 0.2566(10) 0.2598(7)) parameter x = y = zAtomic Displacement Fel Fe2 O Fel Fe2 OParameters (ADP)0.075(4) 0.068(4) 0.019(3) 0.046(3) 0.077(3) 0.01* wR 3.52% 4.31%RF24.96% 4.56%Background terms 8 4Table 1 : Structural details derived from the refinements. * was fixed.

[0098] All samples, except for FG2-200, display a set of diffraction peaks located at 30.7, 36.0, 43.6, 54.0, 57.5, and 63.1°, indexed to (220), (311), (400), (422), (511) and (440) planes of the spinel Fe3O4 phase. The broad peak centered at around 25° corresponds to the (002) plane or the interlayer distance of rGO. However, when annealed / heated at 200 °C, a mixture of phases consisting of Fe3O4, FeOOH, and iron chloride hydroxides (as shown in Fig. 3), forms as a result of incomplete reaction of FeC with GO. The morphology of these FesO^rGO products was characterised by SEM. It can be seen in Fig. 2(b) that FG2-600 presents a structure of high porosity where nanoflakes containing open pores pile loosely to form a hierarchically porous network. These pores on rGO laminates result from the gas emitted by the decomposition of functional groups on GO during the annealing / heating process. Such a porous network provides ample channels for electrolytes and, hence, ions to percolate into the bulk of the composite; in this regard, a higher annealing temperature for FG2-600 supposedly introduces more pores compared to FG2-400 (Fig. 4(a) and 4(b)). Fig. 2(c) shows that Fe3O4 nanoparticles are uniformly dispersed on the rGO surface without any obvious evidence of particle clustering or aggregation. EDX results in Fig. 2(d)-(g) confirm the uniform distribution of Fe3O4 nanoparticles within the composite system.

[0099] Overall, a hierarchically porous rGO matrix with a homogeneous distribution of Fe3O4 nanoparticles is formed, which can provide interconnected ion diffusion channels and adequatebuffer space to resist volume change during cycling. Additionally, the high surface area derived from the 3D rGO layer enables a large contact area between the electrode and electrolyte. To explore the influence of rGO arrangement at the macroscale on the battery performance, 2D FesO^rGO membranes were prepared as described above and compared against the 3D aerogels. Different from the randomly aligned laminates in FesO^rGO aerogels, FesO^rGO membrane (Fig. 4(c)) presents a typical wrinkled paper-like structure of rGO, more compact than its aerogel counterpart. Fig. 4 (d)-(f) evidenced the homogeneous dispersion of Fe3O4 in the membrane.

[0100] The weight ratio of Fe contents in composites ranges from 10 wt% to 29 wt%, as quantitively measured by ICPMS (Fig. 5(a)) and TGA (Table 2 and Fig. 6).G- FG1- FG2- FG2- FG2- FG3- FGM-400 400 200 400 600 400 400Remaining Mass 7.8% 16.5% 23.7% 26.7% 27.6% 28.2% 28.7%RatioTable 2: Remaining weight ratio of samples after applying TGA treatment from room temperature to 800 °C at a ramping rate of 5 °C / min under O2.

[0101] The conductivity of the electrodes was measured using a four-probe method (Fig. 5(b)) after aerogels were compressed in pellet form. Compared with the electrical conductivity of pure rGO aerogel (9.3 S / m), the incorporation of Fe3O4 improved the electrical conductivity of the composite, and the electrical conductivity increased with increasing Fe3O4 loading. The higher porosity of graphene foam may lead to higher activation energy for the conduction process. Hence, despite the low electrical conductivity of Fe3O4 nanoparticles (0.3 S / m), the added Fe3O4 in the rGO framework acts as a physical crosslinker in the aerogel and, therefore, reduces the resistance at junction areas in rGO laminates.

[0102] XPS was performed to examine the variation in functional groups in these composites, as shown in Fig. 5(c) and Fig. 7. Compared with pure rGO aerogel G-400, FG2- 400 shows an increase in C=C C 1s and a decrease in C-0 and C=O C 1s signal, indicating that oxygenated groups on GO act as anchors to produce Fe3O4 nanoparticles. Upon increasing the iron (FeC ) loading, Fe3O4 weight ratio in aerogel composites increases with a slight increase in Cl residue (Fig. 7(a)). On elevating the heating temperature from 400 °C to 600 °C, higher temperature intensifies the thermal decomposition of GO, and thus leaves more Fe3O4 nanoparticles (Fig. 7(b)). Combining with the XPS results, the preparation process of Fe3O4 / rGO aerogels, the schematic of which is demonstrated in Fig. 1 , can thus be clarified as follows. The Fe3O4 / rGO aerogels were prepared via a modified method that involved theaerogel-making from a mixture of GO and an iron salt, and the thermal treatment of such aerogels. When dispersed in water, oxygenated functional groups on GO render GO sheets negatively charged, thus attracting Fe3+cations to be attached. After being transformed into aerogels via freeze-drying, GO was thermally reduced by decomposing oxygen-containing groups into gases under N2 atmosphere, and simultaneously, FeC reacts with oxygencontaining groups on GO, mostly epoxy and hydroxyl groups, leading to the formation of iron oxide. In the hydrothermal treatment, the most common method of producing FesO^rGO composites, rGO sheets tend to agglomerate to lower the surface energy and may restack, which could lead to the inhomogeneous arrangement of Fe3O4 nanoparticles on the sheets. Thus, the present invention allows the formation of a highly porous rGO network and a uniform distribution of Fe3O4 nanoparticles through Fe3+anchoring onto GO sheets and the reduction of GO.Results - electrochemical performance

[0103] The electrochemical behaviour of all samples was examined by performing CV and GOD tests. Fig. 8(a) presents the CV curves of the FG2-600 electrode during the first four cycles in the 0.01-3 V voltage window at a scan rate of 0.1 mV / s. In the first cathodic process, a sharp peak is observed at 0.55 V, which is assigned to the electrochemical conversion reaction of Fe3O4, and the irreversible formation of solid electrolyte interphase (SEI) by electrolyte decomposition. The current response near 0.01 V corresponds to the Li intercalation into rGO sheets. The first anodic process exhibits two broad peaks located at 1.1 V and 1.7 V, which are attributed to the oxidation of Fe° into Fe2+and Fe3+, respectively. The reversible reactions occurring in the battery are expressed as below:C (?'GO) 4.x £r~ 4 4" ££C (2)

[0104] In the subsequent scans, both the reduction and oxidation peaks are shifted to a higher voltage, indicating the transformation of the crystalline iron oxide during the first cycle. It can be observed that CV curves of FG2-600 practically overlapped after the first two cycles, highlighting the excellent cycling reversibility of FG2-600.

[0105] The influence of pore structure on the battery performance of Fe3O4 / rGO composites as anodes was investigated via the controlled variation of the composition and annealing temperature. GCD tests were performed by applying a constant current density of 100 mA / g in the 0.01 V-3 V potential window. Fig. 8(b) presents the galvanostatic cycling profiles of theFG2-600 electrode. The profile, which is representative of all FesC / rGO electrodes (Fig. 9), changes considerably after the first cycle, where the electrolyte decomposition-mediated SEI formation appears in a plateau at around 0.75 V. Afterwards, the profile becomes fully reversible, as evident from the overlaying charge and discharge traces from 2nd to 50th cycle. Nevertheless, a high reversible capacity of 1364 mAh / g is obtained in the 2nd cycle, which drops marginally to 1048 mAh / g after 50 cycles. The high specific capacity of FG2-600 has exceeded the theoretical value for either FesO4 (924 mAh / g) or rGO (744 mAh / g). Indeed, such extra Li storage is commonly observed for transition metal oxide anodes. This may be because the electrochemically reduced Fe particles enable the excessive storage of spin-polarised electrons and result in large surface capacitance. To assess the contribution of the rGO to the specific capacity displayed by the FG2-600 electrode, pure rGO aerogel electrodes were cycled under the same conditions (Fig. 10(a) and (b)). Typically, G-600 delivers an initial charging capacity of 434.6 mAh / g with a Coulombic efficiency of 57.6%, but the capacity rapidly decreases to 247.8 mAh / g after 30 cycles. The low initial Coulombic efficiency owes to the SEI formation during the first discharge (reduction). The subsequent decay in capacity, which is commonly observed in graphene-based electrodes, can be attributed to the restacking of graphene sheets and, hence, the loss of active sites for Li diffusion and storage. Therefore, it can be said that the specific capacity displayed by FG2-600 after 50th cycle primarily originates from the redox cycling of the FesO4 and the capacity loss between the 2nd and the 50th cycle can be attributed to the capacity loss for rGO. However, to evaluate the effect of the relative iron content in the electrode, FGx-400 electrodes with x = 1 , 2 and 3 were galvanostatically cycled. The corresponding cycling data is shown in Fig. 8(c) and Fig. 10. Compared to the bare rGO electrode, all aerogel electrodes loaded with FesO4 show an enhanced specific capacity, and it increases with the increasing weight ratio of FesO4 from FG 1-400 to FG2-400 but decreases for FG3-400. The cycling data in Fig. 8(c) thus indicates that both rGO and FesO4 play a synergistic role in determining Li storage capacity. Among all, FG2-400 delivers not only the highest initial reversible capacity of 1364.4 mAh / g, three times that of the rGO electrode, but also a higher initial Coulombic efficiency of 73.8% (Fig. 10(c)). 2D FesO4 / rGO membranes were also tested to understand the influence of composite microstructure structure on battery performance. It can be seen (Fig. 8(c)) that the specific capacity of FGM is far from that of any aerogel, which demonstrates that 3D aerogel structure at the macroscale is more favourable for the lithiation / de-lithiation process.

[0106] Fig. 8(d) compares the cycling performance of FG2-400 and FG2-600, the two high- performing aerogel electrodes, and sheds light on the role of the microstructure in affecting the electrochemical performance. FG2-600, with a lower Fe loading of 19.2 wt%, despite a lower capacity in the first few cycles, shows a better capacity retention than FG2-400. After 100cycles, FG2-600 displays a capacity of 1053 mAh / g with a retention rate of 86.9% compared to 74.5% retention and 1016.3 mAh / g for FG2-400. With respect to the electronic conductivity, which is higher for FG2-400 than FG2-600, the capacity and cyclability data are counterintuitive. The origin of FG2-600’s better performance lies in its microscopic structure, which was further investigated in detail. The cross-sectional morphology of FesO^rGO aerogels in Fig. 8(e) and (f), and Fig. 11 offer a comprehensive insight into the impact of the annealing process on the resulting composite structure. Clearly, the thickness of the aerogel increases upon increasing the annealing temperature from 400 °C to 600 °C even though the same mass loading and composition were applied in the preparation step. The thickness of the aerogel changes from 68.2 pm for FG2-400 to 207.3 pm for FG2-600, accompanied by a structural transformation from a densely packed arrangement to a loosely built construction, which can be further confirmed by the SEM images at higher magnification. Evidently, it can be seen in Fig. 8(e) and (f) that the stacking of rGO layers is present in FG2-400 while most rGO sheets are exfoliated into fewer layers in FG2-600. This decrease in rGO lamellar structure at elevated temperatures stems from higher pressure generated during the decomposition of oxygenated functional groups at a higher temperature, leaving rGO sheets more exfoliated. This fewer-layered configuration ensures a heightened exposure of electrode materials to the electrolyte and prevents the restacking of rGO layers, thus resulting in an efficient utilization of active materials in battery reactions and a well-maintained structure after cycling. ECSA measurements on FG2-400 and FG-600 in Fig. 8(g) also quantitatively evidence that the availability of electroactive sites exceeds in FG2-600 than that in FG2-400. Apparently, the expanded space in FG2-600 as ion diffusion shortcuts could lead to an unexpected loss in active materials per unit volume. It is typical that such porous electrodes suffer from low tap density, which in turn negatively impacts the volumetric energy density of the electrode. FG2- 200 shows the lowest charge capacity of 753.9 mAh / g in the first cycle, rapidly decays to 434.7 mAh / g after 10 cycles, and then remains stable in the following cycles (Fig. 10(e)). This poor performance can be ascribed to the incomplete reaction between the Fe3+and GO at 200 °C and the mixture of phases as indicated by the XRD pattern (Fig. 3), which are electrochemically less active and less stable for Li+cycling. FG1 samples, which were thermally treated from 200 °C to 600 °C, show a similar capacity trend as that for FG2 samples (Fig. 10(f)).

[0107] Given the enhanced electrochemical performance of FG2-400 and FG2-600, these electrodes were selected to analyse their rate capability, as shown in Fig. 8(h) and (i). At lower current rates, FG2-400 shows a higher capacity than FG2-600. However, the Li storage capacity of FG2-600 exceeds that of FG2-400, when the current density goes above 0.5 A / g. The improved performance of FG2-600 at higher rates is probably because the highly porous structure with interconnected channels allows easy access to the electrolyte and facile Li+diffusion pathways. When testing the durability of electrodes at a higher rate, such as 0.5 A / g and 1 A / g, the electrode suffers from rapid volumetric change, and the resultant stress increases the polarization. As shown in Fig. 8(i), the capacity of FG2-600 initially increases with cycling due to the activation process of iron oxides. After 1000 cycles, FG2-600 delivers a capacity of 615.2 mAh / g and 575.2 mAh / g, with a retention rate of 85.2% and 82.8%, at a current density of 0.5 A / g and 1 A / g, respectively, indicating the excellent structural integrity maintained over the long-term cycling.

[0108] To further understand the electrochemical reaction mechanism, the evolution of FG2- 600 was probed in operando by XRD during the battery cycling. The data is illustrated in Fig. 12(a). As expected for the electrochemical conversion mechanism proposed above, the peaks corresponding to FesO4 quickly fade during the first discharge. After the first cycle, aside from the sharp peaks from the cell body, only the rGO peak is left, indicating that the active FesO4 converts to an amorphous phase, typical of all conversion-type oxide anode materials. Ex situ XRD of the FG2-600 electrode cycled 10 times and 50 times further confirms the operando XRD observation (Fig. 13).

[0109] To gain further insights into the electrode process, the composite electrodes were probed by electrochemical impedance spectroscopy or EIS. Nyquist impedance data of all electrodes, as shown in Fig. 12(b) and (c), consists of a depressed semi-circle in the high-to- medium frequency region, corresponding to the charge transfer process (resistance: RCT) at the electrode-electrolyte interface and an inclined line in the low-frequency region, corresponding to the solid-state diffusion of lithium within active materials. The RCT of all composites is listed in Table 3 and the EIS equivalent circuit is demonstrated in Fig. 14. FGM- 400 possesses an RCT of 704.4 Q, which is higher than its aerogel counterparts. This result demonstrates that the porous aerogel microstructure enables a large electrode-electrolyte contact area and, thus, fast ion transport across the interface. As for aerogel samples, RCT varies with the FesO4 mass loading and the annealing temperature. FG2-400 displays the lowest RCT of 296.6 Q, most likely owing to the combination of its high electronic conductivity and the optimal microstructure, and the RCT is consistent with its galvanostatic performance.FG1- FG2- FG2- FG2- FG3- FGM- FG2- FG2- FG2- FG2-400 200 400 600 400 400 600- 600- 400- 400-10th 50th 10th 50thRct 484.1 564.1 296.6 580.3 679.4 704.4 54.6 40.7 286.3 183.6(Q)Table 3: RCT of electrodes tested at the frequency range from 100 mHz to 1 MHz.

[0110] The comparison between the EIS data of FG2-400 and FG2-600 after cycling, as shown in Fig. 12(d) and (e), respectively, reveals the origin of the stable long-term cycling of FG2-600. The RCT of FG2-600 dramatically reduces from 580.3 Q before cycling to 54.6 Q after 10 cycles and 40.7 Q after 50 cycles, which implies the significantly facilitated redox kinetics. This is believed to result from the activation of iron oxide nanoparticles that are homogeneously distributed in the optimal 3D porous network formed by rGO. Since structural integrity is a pivotal factor in achieving the durability of electrode materials, the morphology of cycled electrodes was probed by SEM and shown in Fig. 12(f) and (g), and Fig. 15. After 50 cycles, the highly porous structure of FG2-600 still maintains, and rGO laminates remain well- exfoliated without obvious evidence of restacking laminates. Fig. 12(f) shows that crystalline FesO4 nanoparticles have most likely changed into an amorphous form as indicated by the XRD patterns of cycled electrodes and consistent with the CV profiles. EDX in Fig. 15 evidences the uniform distribution of Fe after 50 cycles.

[0111] To evidence the excellent battery performance demonstrated by the present invention (for example FG2-600 composite electrode), Fig. 16 compares it against the electrochemical performance of FesOVgraphene-based LIB anodes reported in the literature in terms of the reversible capacity obtained in the 1st and 100th cycle at 100 mA / g. Various forms of FesOVrGO composites, ranging from 0D to 3D, have been studied in search of the optimal structure for Li storage and the electrochemical performance of the present invention is evidently outstanding, especially considering its facile preparation process. This work compared the battery performance of 2D and 3D FesOVrGO electrodes and showed the enhanced Li cycling performance of 3D aerogel due to its flexibility and ample access to active materials. Based on 3D aerogel structure of FG2-400, we further optimized the microstructure (FG2-600) by adjusting the thermal annealing process. The tailored microstructure of FG2-600 clearly has a positive impact on Li cycling especially at high current densities and on maintaining the electrode integrity for long-term cycling.

[0112] Although the invention will be described with reference to specific examples it will be appreciated by those skilled in the art that the invention may be embodied in many other forms.

Claims

CLAIMS:

1. A method of preparing an rGO-based aerogel, the method comprising: a. providing a dispersion comprising GO and a transition metal salt; b. lyophilising the dispersion to obtain a transition metal salt / GO aerogel; and c. heating the transition metal salt / GO aerogel in an inert atmosphere to obtain a transition metal oxide / rGO aerogel or a transition metal / rGO aerogel.

2. The method according to claim 1, wherein the transition metal is selected from Fe, Co, Ni, Or, or any combinations thereof, preferably Fe.

3. The method according to claim 2, wherein the transition metal oxide is selected from FeO, FesO4, Fe2Os, a-Fe2Os, p-Fe20s, Y-Fe2Os, £-Fe2Os, or any combinations thereof, preferably Fe3O4.

4. The method according to any one of claims 1 to 3, wherein the transition metal salt is a halide salt, carbonate, sulphate, acetate, oxalate, or any combinations thereof, preferably a halide salt, more preferably FeC .

5. The method according to any one of claims 1 to 4, wherein the step c comprises heating the transition metal salt / GO aerogel for about 1 to about 10 hours.

6. The method according to any one of claims 1 to 5, wherein a concentration of GO in the dispersion is about 0.1 mg / ml to about 10 mg / ml.

7. The method according to any one of claims 1 to 6, wherein a weight ratio of the transition metal salt and the GO in the dispersion is about 10:1 to about 1:10.

8. The method according to any one of claims 1 to 7, wherein the step b comprises lyophilising the dispersion at a temperature of about -100°C to about -50°C.

9. The method according to any one of claims 1 to 8, wherein the step c comprises heating the transition metal salt / GO aerogel in an atmosphere of an inert gas comprising N2, Ar, Ne, He, or any combinations thereof.

10. The method according to any one of claims 1 to 9, wherein the step c comprises heating the transition metal salt / GO aerogel at a temperature of about 100 °C to about 1000 °C, or about 200 °C to about 600 °C, or above about 200 °C, or above about 300 °C, or above about 400 °C.11 . An rGO-based aerogel obtained from the method according to any one of claims 1 to 10.

12. An electrode material comprising the rGO-based aerogel according to claim 11.

13. An electrode comprising or consisting of the electrode material according to claim 12.

14. The electrode according to claim 13, substantially devoid of a binder.

15. The electrode according to claim 13 or claim 14, wherein the electrode is freestanding.

16. A method of preparing an electrode, comprising the steps of: a. obtaining the rGO-based aerogel according to claim 11 ; b. pressing the rGO-based aerogel into an aerogel film to thereby obtain an electrode17. An electrode obtained by the method according to claim 16.

18. An alkali metal-ion battery comprising the electrode according to any one of claims 13 to 15 or claim 17.

19. The battery according to claim 18, having a specific capacity of about 300 mAh / g to about 2000 mAh / g at a current density of about 0.1 A / g to 5 A / g, or a specific capacity of above about 400 mAh / g at a current density of about 2A / g, or a specific capacity of above about 600 mAh / g at a current density of about 1A / g for over about 1000 charging-discharging cycles, or a specific capacity of about 1100mAh / g after about 100 charging-discharging cycles.

20. The battery according to claim 18 or claim 19, having a capacity retention rate of at least about 60% after about 1000 charging-discharging cycles at a current density of about 1 A / g.

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