Technical Field

The present invention relates to a three-dimensional (3D) printed microlattice. Background

With the increasing demand for light-weight and wearable electronics, energy storage devices with flexibility, stretchability/compressibility and durability are of prime importance. Quasi-solid-state aqueous rechargeable batteries are recognized as one of the outstanding solutions due to their high power density, cycling stability and enhanced safety under various environment or mechanical deformations. To date, various types of aqueous rechargeable batteries have been explored, including alkali- ion (Li ⁺ , Na ⁺ and K ⁺ ) batteries, metal-ion (Al ³⁺ , Zn ²⁺ and Mg ²⁺ ) batteries and nickel- metal (Zn, Co and Fe) batteries. Among them, aqueous nickel-iron (Ni-Fe) batteries have drawn tremendous attention due to: (i) both Ni and Fe bring naturally low cost, non-toxic and non-flammable; (ii) both Ni- and Fe-based active materials being insoluble in alkaline solution, thus do not require separators. However, there is still a need for a flexible and compressible battery which is able to withstand sufficiently high compressibility while providing sufficient power density.

Summary of the invention The present invention seeks to address these problems, and/or to provide an improved material which may be utilised in a battery, particularly a compressible and flexible battery.

According to a first aspect, there is provided a 3-dimensional (3D) printed microlattice comprising multilayers of a graphene-based composite material, wherein the graphene- based composite material comprises graphene oxide/carbon nanotubes. In particular, the microlattice may be a free-standing microlattice.

According to a particular aspect, the graphene oxide/carbon nanotubes may comprise reduced graphene oxide/carbon nanotubes (rGO/CNTs).

The microlattice may be in any suitable form. For example, the microlattice may be in the form of an aerogel. According to a particular aspect, the microlattice may have an average density of 15-25 mg/cm ³ . According to another particular aspect, the microlattice may have a compressive strain of ≤ 60%.

The microlattice may comprise interconnected pores. According to a second aspect, the present invention provides an electrode for a battery comprising the microlattice according to the first aspect and an active electrode material.

The active electrode material may be any suitable active electrode material. For example, the active electrode material may comprise a polycrystalline active electrode material. In particular, the polycrystalline active electrode material may comprise at least a portion having one or more average cross-sectional physical dimension of 50 nm to 200 μm.

The active electrode material may be in any suitable form. For example, the active electrode material may be in the form of, but not limited to: flakes, sheets, rods, or spherical.

The active electrode material may comprise, but is not limited to, metal oxides, metal hydroxides, or a combination thereof. For example, the active electrode material may comprise any one of, but not limited to: Ni(OH) ₂ or Fe ₂ O ₃ .

According to a particular aspect, the electrode may be a cathode and the microlattice may comprise Ni(OH) ₂ as the active electrode material. According to another particular aspect, the electrode may be an anode and the microlattice may comprise Fe ₂ O ₃ as the active electrode material.

The electrode may have a specific capacity of 350-1500 mAh per gram of active electrode material. According to a third aspect, the present invention provides a battery comprising an electrode according to the second aspect. The battery may be any suitable battery. In particular, the battery may be a compressible battery. Even more in particular, the battery may be a quasi-solid state battery. For example, the battery may be a quasi-solid state Ni-Fe battery.

According to a particular aspect, the battery may comprise a gel electrolyte. According to a fourth aspect, there is provided a method of forming a 3-dimensional (3D) printed microlattice comprising multilayers of a graphene-based composite material, the method comprising:

3D printing the multilayers layer-by-layer with an ink comprising a graphene- based composite material to form a 3D printed microlattice, wherein the graphene-based composite material comprises graphene oxide/carbon nanotubes; freeze-drying the 3D printed microlattice to form a 3D printed aerogel microlattice; and heating the 3D printed aerogel microlattice.

According to a particular aspect, the 3D printed aerogel microlattice may comprise reduced graphene oxide/carbon nanotubes (rGO/CNTs) following the heating.

The method may further comprise depositing an active electrode material on the 3D printed aerogel microlattice to form an electrode. The active electrode material may be as described above in relation to the second aspect of the present invention.

The depositing may be by any suitable method. For example, the depositing may be by, but not limited to: chemical solution deposition, or hydrothermal method.

According to a particular aspect, the hydrothermal method may be followed by a heating step. Brief Description of the Drawings

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: Figure 1 shows the viscosity as a function of the shear rate on a log scale for the GO/CNT and GO inks;

Figure 2 shows the storage modulus (G') and loss modulus (G") of GO/CNT and GO inks as a function of shear stress; Figure 3 shows the shear modulus of the GO/CNT and GO inks as a function of frequency;

Figure 4 shows the schematics of the fabrication processes of the 3D printed rGO/CNTs@Ni(OH) ₂ cathode and 3D printed rGO/CNTs@a-Fe203 anode;

Figure 5 shows the mechanical performance of the 3D printed rGO/CNTs micro lattices; Figure 6 shows the mass loading of Ni(OH) ₂ nanoflakes on 2mm-thick 3D printed rGO/CNTs hybrid aerogel lattices with deposition time from 10 to 60 minutes;

Figure 7 shows the XRD pattern of the 3D printed rGO/CNTs@Ni(OH) ₂ microlattices;

Figure 8 shows the mass loading of α- Fe ₂ O ₃ nanorods on the 2mm-thick 3D printed rGO/CNTs hybrid aerogel lattices with deposition time from 5 to 20 hours; Figure 9 shows the XRD pattern of the 3D printed rGO/CNTs@ α- Fe ₂ O ₃ microlattices;

Figure 10 shows a comparison of the nominal stress (force divided by cross-section area of control sample) vs. strain for the samples 3D printed rGO, 3D printed rGO/CNTs, 3D printed rGO/CNTs@Ni(OH) ₂ and 3D printed rGO/CNTs@ α- Fe ₂ O ₃ microlattices; Figure 11 shows the mechanical performance of 3D printed rGO/CNTs@Ni(OH) ₂ until 60 % compression for 10 cycles;

Figure 12 shows the mechanical performance of 3D printed rGO/CNTs@α-Fe ₂ O ₃ until 60 % compression for 10 cycles;

Figure 13(a) shows the areal capacitance of the electrodes measured at 10-100 mA cm ^-2 plotted as a function of electrode thickness; Figure 13(b) shows the areal capacity of 1-4 mm thickness devices obtained at different current densities (10-100 mA cm ^-2 ); Figure 13(c) shows the specific capacity and volumetric capacity plotted as a function of electrode thickness with current density of 30 mA cm ^-2 ; Figure 13(d) shows the specific capacity obtained at different current densities from 3D rGO/CNTs@Ni(OH) ₂ electrodes with different thickness (1-4 mm);

Figure 14(a) shows the areal capacitance of the electrodes measured at 10-100 mA cm ^-2 plotted as a function of electrode thickness; Figure 14(b) shows the areal capacity of 1-4 mm thickness devices obtained at different current densities (10-100 mA cm ^-2 ); Figure 14(c) shows the specific capacity and volumetric capacity plotted as a function of electrode thickness with current density of 30 mA cm ^-2 ; Figure 14(d) shows the gravimetric capacity obtained at different current densities from 3D rGO/CNTs@a- Fe ₂ O ₃ electrodes with different thickness (1-4 mm);

Figure 15(a) shows a comparison of the areal capacity of various 3D printed rGO/CNTs cathodes with different electrode thicknesses; Figure 15(b) shows the CV curves of the 3D printed rGO/CNTs cathodes at different scan rates; Figure 15(c) shows the galvanostatic charging-discharging profiles of the 3D printed rGO/CNTs cathodes at various current densities; Figure 15(d) shows a comparison of the areal capacitance of various 3D printed rGO/CNTs anodes with different electrode thicknesses; Figure 15(e) shows the CV curves of the 3D printed rGO/CNTs anodes at different scan rates; Figure 15(f) shows the galvanostatic charging-discharging profiles of the 3D printed rGO/CNTs anodes at various current densities; Figure 16(a) shows a comparison of the specific capacity of the 4 mm thickness 3D printed rGO/CNTs@Ni(OH) ₂ electrode with the values of previously reported Nickel- based electrodes, nano-Ni(OH) ₂ /C composite, β-Ni(OH) ₂ , P-NiCo ₂ 0 ₄ NWAs, Ni(OH) ₂ /NG, Ni(OH) ₂ /MWNT, GF/CNTs/Ni(OH) ₂ , b-Nί(OH) ₂ /ONR3, CC-CF@NiO, Ni(OH) ₂ /GS; Figure 16(b) shows a comparison of the specific capacity of the 4 mm thickness 3D printed rGO/CNTs@α-Fe ₂ O ₃ electrode with the values of previously reported iron-based electrodes, FeOx/Graphene, Fe NPs@C, GF/CNTs/Fe ₂ O ₃ , CC- CF@Fe ₃ O ₄ , FeOx/CNTs, Graphene/Fe ₃ O ₄ paper, Fe ₂ O ₃ -15, Fe ₃ O ₄ -rGO, Fe ₂ O ₃ NTs@CC, Fe ₂ O ₃ N Ps/Graphene;

Figure 17 shows the electrochemical characterizations of the electrodes, on the left column 3D printed rGO/CNTs@Ni(OH) ₂ microlattices cathode, and on the right column 3D printed rGO/CNTs@α-Fe ₂ O ₃ microlattices anode; Figures 17(a) and (d) show the cyclic voltammetry curves of at different scan rates from 5 to 40 mV/s; Figures 17(b) and (e) show the galvanostatic charging-discharging curves of at various current densities; Figures 17(c) and (f) show the capacity retention of both 3D printed electrodes tested at a scan rate of 200 mA cm ^-2 for 5000 cycles; Insets of Figures 17(c) and (f) show the corresponding CV curves at the first cycle and the 5000 ^th cycle; Figure 18 shows a plot comparing the areal capacity of 3D printed rGO/CNTs@Ni(OH) ₂ and 3D printed rGO/CNTs@α-Fe ₂ O ₃ electrodes with the values of previously reported electrodes with high mass loading, Mnx-h, Ni-HAB MOF, CNT/V ₂ O ₅ , PPy, Graphene, M0O3;

Figure 19(a) shows a schematic illustration of a Ni-Fe battery device tested in aqueous electrolyte; Figure 19(b) shows a comparison of CV curves of 3D printed rGO/CNTs@Ni(OH) ₂ cathode and 3D printed rGO/CNTs@α-Fe ₂ O ₃ anode at 30 mV s ^-1 ; Figure 19(c) shows the CV curves at different scan rates from 5 to 50 mV s ^-1 ; Figure 19(d) show the galvanostatic charge-discharge curves of the Ni-Fe battery in aqueous electrolyte at various current densities; Figure 19(e) shows the cycling performance of the Ni-Fe battery for 15,000 cycles at a current density of 300 mA cm ^-2 , with the inset showing the charge/discharge curves for the first five cycles and the last five cycles, respectively;

Figure 20(a) shows a schematic illustration of a 3D printed QSS-NFB device tested in gel electrolyte; Figure 20(b) shows the CV curves of the 3D printed QSS-NFB device in gel electrolyte at different scan rates from 5 to 50 mV s ^-1 ; Figure 20(c) shows the galvanostatic charge-discharge curves of the 3D printed QSS-NFB device in gel electrolyte at various current densities from 10 to 300 mA cm ^-2 ; Figure 20(d) shows the rate properties of the 3D printed QSS-NFB device in gel electrolyte at various current densities; Figure 20(e) shows the cycling performance of the 3D printed QSS-NFB device with gel electrolyte for 10,000 cycles at a current density of 300 mA cm ^-2 , with the inset showing the charge/discharge curves for the first five cycles and the last five cycles, respectively;

Figure 21(a) shows a comparison of the areal capacitance of various 3D printed QSS- NFB devices with different cell thicknesses; Figure 21(b) shows a comparison of the volumetric capacitance of various 3D printed QSS-NFB devices with different cell thicknesses; Figure 21(c) shows the areal capacity and specific capacity comparison of the 3D printed QSS-NFB devices with previous studies; Figure 21(d) shows Ragone plots of the 3D printed QSS-NFB device, with values reported for other energy storage devices added for comparison;

Figure 22(a) shows the compression stress vs. strain curves of a 3D printed compressible QSS-NFB-8 device with maximum strain of 80 % under compressive deformation conditions; Figure 22(b) shows the mechanical performance of 3D printed compressible QSS-NFB-8 device under 60 % compression for the 1 ^st and 10 ^th cycles; and

Figure 23(a) shows the capacity retention of a representative 3D printed compressible QSS-NFB device at different strains, and the insert shows the result for CV curves; Figure 23(b) shows the galvanostatic charge/discharge curves of a representative 3D printed compressible QSS-NFB device at different strains, with current density at 30 mA cm ^-2 ; Figure 23(c) shows the Nyquist impedance plot of the 3D printed compressible QSS-NFB device at different strains with frequency ranging from 10 kHz to 0.1 Hz; Figure 23(d) shows the capacity retention as a function of compressed cycle number, and the insert shows the electrical resistance change when repeatedly compressed up to 60% of strain for over 10 cycles; Figure 23(e) shows the cycling performance at different compression states with current density at 200 mA cm ^-2 .

Detailed Description

As explained above, there is a need for an improved material for use in flexible and compressible batteries.

In general terms, the present invention provides a material for use in flexible and compression-tolerant and flexible electronics which has high specific capacity, as well as high areal and volumetric capacities. In particular, the material may be used for forming electrodes and enable ultrahigh active electrode material loading while remaining highly compressible. This enables the electrodes formed from the material of the present invention to be used in flexible and compressible batteries, which may in turn be used in stress-tolerant flexible/wearable electronic devices.

According to a first aspect, there is provided a 3-dimensional (3D) printed microlattice comprising multilayers of a graphene-based composite material, wherein the graphene- based composite material comprises graphene oxide/carbon nanotubes. In particular, the graphene oxide/carbon nanotubes may comprise reduced graphene oxide/carbon nanotubes (rGO/CNTs). Even more in particular, the rGO/CNTs may comprise 1- dimensional CNTs and 2-dimensional rGO nanosheet.

The microlattice may be in any suitable form. For example, the microlattice may be in the form of an aerogel. For the purposes of the present application, the term aerogel may be defined as a low-density and highly porous structure with a very high surface area.

According to a particular aspect, the microlattice may have an average density of 15-25 mg/cm ³ . For the purposes of the present invention, the term average density refers to the average volumetric mass density of the microlattice. In particular, the microlattice may have an average density of 16-24 mg/cm ³ , 17-23 mg/cm ³ , 18-22 mg/cm ³ , 19-20 mg/cm ³ . Even more in particular, the microlattice may have an average density of about 20 mg/cm ³ .

The microlattice may have any suitable dimension. The term dimension may refer to the thickness, shape, size of any one edge of the microlattice. In particular, the microlattice may have a suitable thickness. The thickness of the microlattice may be controlled based on the number of layers of the graphene-based composite material comprised in the microlattice. For example, the thickness may be 0.1-4 cm. In particular, the thickness may be 0.5-3.8 cm, 0.7-3.5 cm, 1.0-3.2 cm, 1.2-3.0 cm, 1.5-2.8 cm, 1.7-2.5 cm, 2.0-2.2 cm. The microlattice may have a suitable width. For example, the width of the microlattice may be 600-800 μm.

The microlattice may comprise interconnected pores. In particular, the microlattice may comprise a hierarchical architecture. For example, the microlattice may have a porosity of > 70%. In particular, the porosity may be 70-98%, 75-95%, 80-90%, 85-88%. Accordingly, the microlattice comprises open and hierarchical pores, thereby providing multiple channels for ionic diffusion.

According to a particular aspect, the microlattice may comprise an interconnected porous framework comprising randomly oriented layers with wrinkled morphology. The cross-section of the microlattice may comprise layers which may be dense and may be stacked compactly. In particular, the microlattice may comprise orthogonal multilayers with parallel, porous cylindrical rods. The microlattice may be a free-standing microlattice. In this way, the microlattice may function as a current collector without requiring any additional current collector such as copper or aluminium foil..

According to a particular aspect, the microlattice may be compressible. For the purposes of the present invention, compressible may be defined as the micro lattice’s ability to recover its original shape quickly after compression-release without plastic deformation and without damaging its structure. According to another particular aspect, the microlattice may have a compressive strain of £ 60%.

The 3D printed microlattice may be formed by 3D printing. In particular, any suitable 3D printing method may be used for forming the 3D printed microlattice. Even more in particular, the 3D printing method may comprise direct ink writing.

According to a second aspect, the present invention provides an electrode for a battery comprising the microlattice according to the first aspect and an active electrode material. For the purposes of the present invention, an active electrode material may be defined as any material in an electrode that takes part in electrochemical reactions which store and/or deliver energy in a battery.

The active electrode material may be any suitable active electrode material. According to a particular aspect, the active electrode material may comprise a polycrystalline active electrode material. For example, the active electrode material may have a polycrystalline structure selected from, but not limited to: rhombohedral, tetragonal, cubic, spherical.

The polycrystalline active electrode material may comprise suitable dimensions. For example, at least a portion of the polycrystalline structure of the active electrode material may have one or more average cross-sectional physical dimension of 50 nm to 200 μm. The dimension may refer to at least one of average diameter, height, width or length of the polycrystalline structure. In particular, the polycrystalline structure of the active electrode material may have one or more average cross-sectional physical dimension of 50 nm-200 μm, 75 nm-175 μm, 100 nm-150 μm, 150 nm-100 μm, 200 nm-75 μm, 250 nm-50 μm, 500 nm-25 μm, 750 nm-1000 nm, 800-900 nm. In particular, the active electrode material comprised in the electrode may be in the nanoscale.

The active electrode material may be in any suitable form. For example, the active electrode material may be in the form of, but not limited to: flakes, sheets, rods, spherical.

The active electrode material may comprise, but is not limited to, metal oxides, metal hydroxides, or a combination thereof. For example, the active electrode material may comprise a metal-organic framework material. In particular, the active electrode material may comprise any one of, but not limited to: Ni(OH) ₂ or Fe ₂ O ₃ . The electrode may comprise a suitable amount of active electrode material. For example, the mass loading of the active electrode material in the electrode may be 20- 160 mg/cm ³ , 50-150 mg/cm ³ , 75-125 mg/cm ³ , 90-110 mg/cm ³ , 95-100 mg/cm ³ . In particular, the mass loading may be ≥ 130 mg/cm ³ . Even more in particular, the mass loading may be 130-160 mg/cm ³ . The active electrode material may be formed directly on the microlattice, thereby retaining the morphological advantages provided by the microlattice as described above in relation to the first aspect. In this way, there may be strong covalent coupling between the active electrode material and the carbon within the electrode, leading to rapid electron transfer kinetics. Further, the interconnected hierarchical pores provide unobstructed channels for ionic transport from bottom-up to surface of the electrodes. The junctions of adjacent filaments and close-knitted contacts due to compact packing order within the microlattice ensures multitude of ionic and electronic pathways. In particular, the electrode may have a specific capacity of 350-1200 mmh per gram of active electrode material. For example, specific capacity may be 400-1000, 500-900, 550-850, 600-800, 650-750, 700-725 mAh per gram of active electrode material.

The electrode may have improved mechanical performance as compared to the microlattice from which it is formed. According to a particular aspect, the compressive stress of the electrode comprising the active electrode material may be higher as compared to the compressive stress of the microlattice without the active electrode material. In particular, the compressive stress of the electrode may be double that of the microlattice without the active electrode material when subjected to identical compression strain. Accordingly, the active electrode material provides a buffer under mechanical influence and improves the robustness of the electrode.

According to a particular aspect, the electrode may be a cathode and the microlattice may comprise Ni(OH) ₂ as the active electrode material. The electrode may comprise Ni(OH) ₂ nanoflakes. In particular, the Ni(OH) ₂ may have a rhombohedral polycrystalline structure. The electrode may have a mass loading of Ni(OH) ₂ of 25-132 mg/cm ³ . The Ni(OH) ₂ cathode may have a specific capacity of 370-380 mAh per gram of Ni(OH) ₂ .

According to another particular aspect, the electrode may be an anode and the microlattice may comprise Fe ₂ O ₃ as the active electrode material. In particular, the Fe ₂ O ₃ may be in any suitable form. For example, the Fe ₂ O ₃ may be α-Fe ₂ O ₃ . The electrode may comprise Fe ₂ O ₃ nanorods. In particular, the Fe ₂ O ₃ nanorods may be holey Fe ₂ O ₃ nanorods. The Fe ₂ O ₃ may have a tetragonal polycrystalline structure. The electrode may have a mass loading of Fe ₂ O ₃ of 46-152 mg/cm ³ . The Fe ₂ O ₃ anode may have a specific capacity of 480-3490 mAh per gram of Fe ₂ O ₃ . According to a third aspect, the present invention provides a battery comprising an electrode according to the second aspect.

The battery may be any suitable battery. In particular, the battery may be a compressible battery. Even more in particular, the battery may be a solid state battery or a quasi-solid state battery. According to a particular aspect, the battery may comprise an electrolyte. The electrolyte may be any suitable electrolyte. For example, the electrolyte may be an aqueous electrolyte or a gel electrolyte. In particular, the electrolyte may be a gel electrolyte.

The electrolyte may be, but not limited to, ionic electrolytes, polymer-based electrolytes, or a combination thereof. In particular, the electrolyte may be a blend of a polymer-based and ionic electrolyte. For example, the electrolyte may be, but not limited to, PVA/KOH, PVA/K ₂ SO ₄ , or a combination thereof.

According to a particular aspect, the battery may be a quasi-solid state Ni-Fe battery. In particular, the battery may comprise a gel electrolyte, a cathode as described above comprising Ni(OH) ₂ as the active electrode material and an anode as described above comprising Fe ₂ O ₃ as the active electrode material.

The battery may have a suitable specific capacity. For example, the specific capacity may be 100-350 mA h/g. In particular, the specific capacity of the battery may be 110- 300, 120-250, 150-200, 175-190 mA h/g. Even more in particular, the specific capacity of the battery may be about 200-220 mA h/g.

The battery may have a suitable capacity retention after multiple cycles. For example, the capacity retention of the battery may be ≥ 80% after 10000-20000 cycles. In particular, the battery may have capacity retention of 90-95% after 10000 cycles, and capacity retention of 80-90% after 15000 cycles.

The battery of the present invention is also able to retain a high percentage of its capacity after undergoing compressive strain, and at the same time recover to its original state. In particular, the battery may have a compressive strain of ≤ 66%. In particular, the battery may have a capacity limit of 300 mA h/g. The battery of the present invention may be used in any suitable application. For example, the battery may be used on its own, or may be combined with multiple units of the battery. The multiple units of batteries may be arranged in series, in parallel, or in combination of the two to obtain the required power or capacity (voltage and/or current, respectively) for any specific application. According to a fourth aspect, there is provided a method of forming a 3-dimensional (3D) printed microlattice comprising multilayers of a graphene-based composite material, the method comprising:

3D printing the multilayers layer-by-layer with an ink comprising a graphene- based composite material to form a 3D printed microlattice, wherein the graphene-based composite material comprises graphene oxide/carbon nanotubes; freeze-drying the 3D printed microlattice to form a 3D printed aerogel microlattice; and heating the 3D printed aerogel microlattice. The ink used in the 3D printing may be any suitable ink. For example, the ink may comprise graphene oxide/carbon nanotubes. The ink may be formed by any suitable method. In particular, the ink may be formed by mixing GO with CNTs. Even more in particular, the ink may be formed by mixing CNT paste in GO suspension to form GO/CNT ink. The CNT paste may comprise purified CNTS. The purified CNTs may be formed by any method known in the art.

The proportion of the GO to CNT in the ink may be any suitable proportion. For example, the ink may comprise 50-85 wt % of GO. In particular, the ink may comprise 55-80 wt %, 60-75 wt %, 65-70 wt % GO. The ink may be formed under suitable conditions. For example, the ink may be formed with stirring. In particular, the stirring may be constant and vigorous to ensure that the CNT paste is well distributed within the GO suspension to form the GO/CNT ink. The ink may be subjected to further steps prior to the 3D printing. For example, the ink may be centrifuged at high speed to remove water within the GO/CNT ink prior to the 3D printing.

The 3D printing may comprise any suitable 3D printing method. For example, the 3D printing may comprise direct ink writing.

The 3D printing may be performed under suitable conditions. For example, the 3D printing may be performed by subjecting the ink dispenser dispensing the ink to a suitable pressure, and dispensing the ink at a suitable speed.

The 3D printing may be performed for a suitable period of time. The time for performing 3D printing may depend on the dimensions of the patterns required. In particular, the 3D printing may be for 5-30 minutes.

The freeze-drying may be under suitable conditions. The freeze-drying may be performed as soon as possible following the 3D printing to prevent drying or cracking of the microlattice from the evaporation of water.

The freeze-drying may be for a suitable period of time and at a suitable temperature to form the aerogel microlattice. The freeze drying may remove the water from the mixture and creates void spaces in the resultant aerogel. The freeze-drying may be for 24-48 hours. Even more in particular, the freeze-drying may be for about 48 hours. The heating the 3D printed aerogel microlattice may be under suitable conditions. For example, the heating may comprise annealing the aerogel. The heating may be in the presence of an inert gas. In particular, the heating may be in the presence of argon gas. The heating may be at a suitable temperature. According to a particular aspect, the heating may be at a temperature of 500-700°C. In particular, the heating may be at a temperature of 525-675°C, 550-650°C, 575-625°C, 600-610°C. Even more in particular, the heating may be at a temperature of about 650°C.

The heating may be for a suitable period of time. For example, the heating may be for 1-6 hours. In particular, the heating may be for 1.5-5.5 hours, 2-5 hours, 2.5-4.5 hours, 3-4 hours. Even more in particular, the heating may be for about 3 hours.

According to a particular aspect, the 3D printed aerogel microlattice may comprise reduced graphene oxide/carbon nanotubes (rGO/CNTs) following the heating. In particular, the heating may convert the GO to reduced GO (rGO). The method may further comprise depositing an active electrode material on the 3D printed aerogel microlattice to form an electrode. The active electrode material may be any suitable material. For example, the active electrode material may be as described above in relation to the second aspect of the present invention.

In particular, the active electrode material may be a suitable material to form a cathode. For example, the active electrode material may be, but not limited to, Ni(OH) ₂ .

The active electrode material may be a suitable material to form an anode. For example, the active electrode material may be, but not limited to, Fe ₂ O ₃ . In particular, the active electrode material may be α- Fe ₂ O ₃ .

The depositing may be by any suitable method. For example, the depositing may be by, but not limited to: solution deposition, chemical solution deposition, or hydrothermal method.

According to a particular aspect, the depositing may comprise chemical solution deposition to deposit an active electrode material to form a cathode. The depositing may be under suitable conditions. For example, the depositing may be at a suitable temperature. In particular, the depositing may be at room temperature. The depositing may be for a suitable period of time. For example, the depositing may be for 10-60 minutes. In particular, the depositing may be for 15-45 min, 20-40 min, 25-35 min. Even more in particular, the depositing may be for 30 min. The chemical solution deposition may comprise dipping the 3D printed aerogel microlattice in a solution comprising the active electrode material. The chemical solution deposition may comprise dipping the microlattice in the solution comprising the active electrode material for a suitable period of time to provide the desired mass loading of the active electrode material on the microlattice. For example, the mass loading may be 25-132 mg/cm ³ .

According to a particular aspect, the depositing may be by hydrothermal method to deposit an active electrode material to form an anode. The depositing may be under suitable conditions. For example, the depositing may be at a suitable temperature. In particular, the depositing may be at a temperature of 50-80°C. Even more in particular, the depositing may be at a temperature of about 60°C.

The depositing may be for a suitable period of time. For example, the depositing may be for 1-20 hours. In particular, the depositing may be for 2-18 hours, 5-15 hours, 7-10 hours. Even more in particular, the depositing may be for 1 hour.

The depositing may further comprise drying the microlattice comprising the active electrode material under suitable conditions, such as time and temperature. For example, the drying may be at room temperature or in an oven. The drying may be for 1-48 hours. In particular, the drying may be at room temperature for 48 hours.

According to a particular aspect, when the depositing comprises the hydrothermal method, the depositing may comprise a heating step. The heating may be any suitable type of heating. In particular, the heating may comprise annealing. The heating may be in the presence of an inert gas. In particular, the heating may be in the presence of argon gas.

The heating may be at a suitable temperature. According to a particular aspect, the heating may be at a temperature of 400-600°C. In particular, the heating may be at a temperature of 420-575°C, 450-550°C, 475-525°C, 500-510°C. Even more in particular, the heating may be at a temperature of about 450°C.

The heating may be for a suitable period of time. For example, the heating may be for 1-5 hours. In particular, the heating may be for 1.5-4.5 hours, 2-4 hours, 2.5-3.5 hours, 2.75-3 hours. Even more in particular, the heating may be for about 2 hours.

The conditions of the hydrothermal method may be adjusted to provide the desired mass loading of the active electrode material on the microlattice. For example, the mass loading may be 46-152 mg/cm ³ .

The present invention will now be exemplified by the following non-limiting examples. Example 1

Evaluation of printing inks

3D printed GO/CNTs hybrid aerogel served as the scaffold for fabrication of the anode and cathode electrodes for Ni-Fe batteries.

The rheology properties of the GO/CNTs ink were evaluated to examine its printing feasibility. Graphene oxide (GO) ink without carbon nanotubes (CNTs) was also prepared as a comparison. CNT paste was dispersed in the dilute GO suspension having a volume of 100 mL and concentration of 20 mg/ mL GO such that the amount of CNT in the mixture is 20 wt %, followed by strong stirring to achieve uniform distribution of CNTs within the GO suspension, and then via high speed centrifuge at 12,000 rpm to remove proper amount of water. The obtained GO/CNTs mixed ink was loaded into a 10mL syringe with a 80 μm nozzle and dispensed into fine lines in a layer-by-layer arrangement on a glass substrate by adjusting the compressed air pressure to 2 MPa and printing rate of 0.1 cm/s. Due to the viscoelastic behaviour of the ink, multiple extruded GO/CNTs layers were stacked together to create complex 3D printed architectures such as microlattices. Conversely, the high concentration GO ink without the addition of CNTs did not have the proper rheology and composition to print and retain fine filaments.

Detailed evaluations of rheology properties of the two inks were investigated under ambient temperature. Figure 1 shows the viscosity as a function of the shear rate on a log scale for the GO/CNT and GO inks. Both inks demonstrated a strong shear-thinning behaviour, wherein the apparent viscosity decreased at the high shear rate, suggesting the inks could flow through a fine nozzle under extruding. The GO/CNT mixed ink with a high concentration exhibited a higher viscosity over the shear rate range compared to the GO ink with a low concentration. GO/CNT ink exhibited a higher viscosity over the shear rate range compared to the GO ink. Meanwhile, the viscosity goes towards infinity around the low shear-rate, indicating that the inks behaved as a Bingham plastic with yield stress, which enabled the ink to return to a zero-shear condition and retain the filament shape once the applied stress was removed.

Figure 2 showed the storage modulus (G') and loss modulus (G") of GO/CNT and GO inks as a function of shear stress. It can be seen that both inks with different viscosities exhibit different yield stress. The yield stress (T _y ) of the GO/CNT and GO inks was about 100 and 10 Pa, respectively. The G' and G" of the GO/CNT ink were much higher than those of the GO ink, where the plateau values of G' were about 6000 and 1500 Pa for the GO/CNT and GO inks, respectively. The higher G' indicated a stiffer character of the GO/CNT ink, which was desirable for printing self-supported complex structures without deformation.

Figure 3 shows the shear modulus of the GO/CNT and GO inks as a function of frequency. The oscillatory frequency sweeps of the GO/CNT and GO inks both display that the G' was larger than G" within the entire frequency interval tested, indicating that both inks exhibited the viscoelastic behaviour with typical gel at rest. The frequency- independent modulus suggested the existence of cohesive interaction within the 3D structure, enabling long-term dispersive stability in static state. Thus, the structure strength and yield stress are substantial for the overall structural robustness.

Fabrication of electrodes

Figure 4 shows a schematic representation of the fabrication processes of 3D printed rGO/CNTs@Ni(OH) ₂ cathode and 3D printed rGO/CNTs@α-Fe ₂ O ₃ anode. First, 3D GO/CNTs microlattices were printed by direct ink writing method using a homogeneous gel blend containing graphene oxide (GO) suspension and purified carbon nanotubes (CNTs) paste. Second, the microlattices were lyophilised to aerogels, followed by post- annealing converting GO to reduced graphene oxide (rGO). Finally, ultrathin Ni(OH) ₂ nanosheet and holey α- Fe ₂ O ₃ nanorod arrays were synthesized onto the aerogel microlattices, by solvothermal method.

Preparation of GO/CNTs Ink

Graphene oxide (GO) nanosheets are prepared from natural graphite flakes (>99.8%, Alfa Aesar) according to the modified Hummer's method. Modified multi-walled carbon nanotubes paste (CNTs Content: 10.9 wt%, Slurry medium: Deionized water) was purchased from Chengdu Organic Chemicals Co. Ltd. In a typical GO/CNTs ink preparation procedure, a proper amount of CNTs paste was dispersed in the dilute GO suspension (100 mL, 20 mg/mL) such that the amount of CNT in the mixture is 20 wt %, followed by strong stirring to achieve uniform distribution of CNTs within the GO suspension, and then via high speed centrifuge at 12,000 rpm to remove proper amount of water. Finally, the GO/CNTs ink was loaded into a 10 mL syringe barrel (EFD) and squeezed to remove air bubbles, after which the ink was extruded through a micro printing nozzles (inner diameter: D = 0.08 mm) to pattern 3D structures on glass substrate.

The rheological properties of as-prepared inks were analysed using stress-controlled HR-2 Discovery hybrid rheometer (TA Instruments).

Preparation of 3D printed rGO/CNTs hybrid aerogel microlattices

The GO/CNTs ink was printed into fine lines in a layer-by-layer arrangement to form orthogonal multilayers with parallel, porous cylindrical rods. A modified YS-D331-X automatic glue dispenser (Shenzhen Yuanshang Automation Technology Co., Ltd.) was used. For direct ink writing, the syringe barrel was attached by a luer-lock to a smooth-flow tapered nozzle whose inner diameter (D) is 80 μm, and the 3D models of various constructs were loaded by the CAD/CAM software, which translated this information into g-code to coordinate the movement of the pneumatic syringe. The ink was then extruded by means of an air-powered fluid dispenser (YS-982M-1, EFD) which provided a constant extruding pressure of 2.0 bar for writing and the writing speed was kept at 2.0 mm/sec for all the 3D printed structures. The diameter of the cylindrical rods equalled the diameter of nozzle and the centre-to-centre rod spacing (L) of 1.0 mm. The height of the 3D printed GO/CNTs ink with regular cubic lattices was varied from 0.3 mm to 6.0 mm and the layers were stacked on the structure such that each layer had a z-spacing of 0.15 mm. The actual printing time, dependent on the dimensions of the patterns and printing speed, ranged from approximately 5 to 30 minutes.

To avoid drying or cracking due to evaporation of water, soon after printing, the 3D printed structures were then frozen in liquid nitrogen and subsequently freeze-dried for 48 hours to form aerogels.

Thereafter, the as-printed 3D GO/CNTs hybrid aerogel microlattices were annealed in a tube-type furnace under argon gas at 650°C for 3 hours with a uniform heating and cooling rate of 2.5°C min ^-1 to form 3D printed rGO/CNTs hybrid aerogel microlattices. For comparison, non-3D printed rGO/CNTs hybrid aerogels were also prepared in similar manner, except without employing 3D printing process.

After freeze-drying and annealing treatments, the 3D printed rGO/CNTs microlattices manifested ultralow volume mass density of about 20 mg cm- ³ with ultralight feature. The 3D printed rGO/CNTs architecture exhibited an interconnected porous framework which consisted of randomly oriented sheets with wrinkled morphology. The cross section of rGO/CNTs layer multilayer microlattices were densified and compactly stacked, ensuring continuous electron transport resonate by the wires-on-sheet structure.

The 3D-printed rGO/CNTs microlattices were subjected to dynamic mechanical analyzer (DMA). As seen in Figure 5, the 3D printed rGO/CNTs showed a maximum strain of about 60% without permanent deformation or cracking, and were able to fully recover after release from the stress, indicating excellent compressibility and resilience. Additionally, the homogeneous GO/CNTs gel inks could easily be printed into various morphologies and dimensions, such as circle, square, vertical letters, with precision. Synthesis of 3D printed rGO/CNTs@Ni(OH) ₂ cathode

Ultrathin Ni(OH) ₂ nanoflake arrays were grown on the 3D printed rGO/CNTs hybrid aerogel microlattices under room temperature by water bath method. The 3D printed rGO/CNTs hybrid aerogel microlattices were wholly immersed into the homogeneous solution mixture with 2.0 g of NiSO ₄ ·6H ₂ O, 0.5 g of K ₂ S ₂ O ₈ , and 1.25 mL of ammonia aqueous solution (NH ₃ ·H ₂ O, 28 wt. %) for different time periods. For the 2 mm-thick 3D printed rGO/CNTs hybrid aerogel microlattices (1x 1x0.2 cm ³ , about 20 mg cm- ³ ), the immersion time was varied from 10 minutes to 60 minutes to obtain a mass loading of Ni(OH) ₂ nanoflakes from 25 to 132 mg cm ^-3 . For the 3D printed rGO/CNTs hybrid aerogels with thickness of 1.0, 2.0, 3.0 and 4.0 mm, the Ni(OH) ₂ nanoflakes growth time was increased to 15, 30, 45 and 60 minutes, respectively.

Thereafter, the samples were taken out, washed with deionized (Dl) water to remove unwanted residuals, and then dried overnight at 60 °C.

Synthesis of 3D printed rGO/CNTs@α-Fe ₂ O ₃ anode

The 3D printed rGO/CNTs@α-Fe ₂ O ₃ anodes were prepared via a facile, seed-assisted, low temperature hydrothermal growth process, followed by annealing treatment. The 3D printed rGO/CNTs hybrid aerogel microlattices were submerged into an aqueous solution of iron nitrate (Fe(NO ₃ ) ₃ , 0.04 M), and then kept in the oven at 50°C for 2 hours.

Thereafter, the samples were taken out, washed with Dl water and ethanol to remove any residuals, and then dried at 60°C for 12 hours. Subsequently, the 3D printed rGO/CNTs@Fe-based seeds hybrid aerogel microlattices were put into a Teflon-lined stainless-steel autoclave containing the aqueous solution mixture with 0.432 g of FeCl ₃ ·6H ₂ O and 0.864 g of NaNO ₃ , which was subsequently maintained at 60°C for different time periods. For the 2 mm-thick 3D printed rGO/CNTs hybrid aerogel microlattices (1x 1x0.2 cm ³ , about 20 mg cm ^-3 ), the growth time was varied from 5 hours to 20 hours to obtain a mass loading of α- Fe ₂ O ₃ nanorod from 46 to 152 mg cm ^-3 .

After cooling down to room temperature, the samples were removed, washed with distilled water, and dried in oven at 60°C overnight. The samples were then thermally annealed at 450°C in argon for 2 hours to obtain porous α- Fe ₂ O ₃ nanorod arrays grown on the surface of the 3D printed rGO/CNTs hybrid aerogel microlattices.

Characterisation of electrodes

Morphology, size, and crystal structure

The morphologies and size studies of the electrode samples were characterized by field-emission scanning electron microscopy (FE-SEM, JSM-7600F, JEOL, Japan). The elemental composition and elemental mapping of the samples were analyzed by energy-dispersive X-ray spectroscope (EDS) attached to the FESEM instrument. The morphologies and structures of the samples were investigated by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEOL, JEM-21 OOF, Japan). The crystal structures of the samples were identified by X-ray powder diffraction (XRD) using a D8 Focus (Germany, Bruker) automated X-ray diffractometer system with Cu- Ka ( λ = 1.5406 Å) radiation at 40 kV. Raman spectra were recorded using a confocal Raman system (WITec Instruments Corp., Ulm, Germany) equipped with the 532 nm laser source. The overall microlattice morphology of the architecture was well-maintained after the growth of interconnecting Ni(OH) ₂ , where the latter exhibited nanoflakes of hundreds of nanometer in diameter and several nanometers in thickness. The synthesis of Ni(OH) ₂ involved every part of the nanostructure with very high tunability via the deposition time (10-60 minutes, from 25 to 132.5 mg cm ^_3 respectively) (Figure 6). EDS maps manifest uniform distribution of Ni and O elements on rGO/CNTs nanosheets, further revealing the presence polycrystalline Ni(OH) ₂ nanoflakes on the rGO/CNT nanosheets. TEM images collected from Ni(OH) ₂ nanostructure also verified the nanoflake morphology grown on rGO/CNTs nanosheets, and well-resolved lattice fringes of 0.26 nm was observed, corresponding to the (101) plane of a-Ni(OH) ₂ (JCPDS #38-0715). Meanwhile, the corresponding selected-area electron diffraction (SAED) pattern indicated that the Ni(OH) ₂ nanoflakes were polycrystalline. XRD analysis further confirmed the formation of rhombohedral Ni(OH) ₂ . As seen in Figure 7, all the peaks were indexed to rhombohedral a-Ni(OH) ₂ except the crystalline peaks at 26.4° and 44.3° that resulted from the rGO/CNTs substrate. As for the rGO/CNTs@α-Fe ₂ O ₃ anode, the spindle-shaped α-Fe ₂ O ₃ nanorod arrays were uniformly deposited throughout the rGO/CNTs microlattices with diameter 200- 300 μm. Similarly, the density of α-Fe ₂ O ₃ nanorods was adjusted by varying the growth time from 0 to 20 hours (46-152 mg cm ^-3 , respectively), as shown in Figure 8. TEM images identified the mesoporous structure of the Fe ₂ O ₃ nanorods with a diameter of 20-30 nm and a length ranging from 0.2-0.5 μm, with pore size of about 5-10 nm. The selected area electron diffraction (SAED) pattern indicated that Fe ₂ O ₃ nanorods were polycrystalline. The well-defined lattice fringed with distances of 0.25 nm correspond to the d-spacings of (110) plane. Figure 9 shows the XRD pattern of the as-prepared rGO/CNTs@α-Fe ₂ O ₃ , and the characteristic peaks in XRD can be well indexed as tetragonal α- Fe ₂ O ₃ (JCPDS 80-2377) (Kong et. al. , 2016).

Mechanical performance Compressive stress-strain tests were performed on a Q800 Dynamic Mechanical Analyzer (Q800 DMA, TA Instruments) at room temperature (25°C). The weight of as- synthesized samples was precisely recorded before and after reaction using an A&D analytical balance (GR-202, Japan) to calculate the actual catalyst loading amount.

Figure 10 shows the mechanical performance in uniaxial compress tests of the 3D printed rGO, rGO/CNTs, rGO/CNTs@Ni(OH) ₂ and rGO/CNTs@α-Fe ₂ O ₃ micro lattices. After growth of Ni(OH) ₂ or α- Fe ₂ O ₃ , the compress stress of the 3D rGO/CNTs@Ni(OH) ₂ and rGO/CNTs@α-Fe ₂ O ₃ microlattices increased by about 2 times when subjected to identical compression strain (60%), indicating that the grown active electrode materials acted as buffers under mechanical influence with considerable robustness enhancement. Meanwhile, the compressive elasticity of the 3D printed rGO/CNTs- based microlattices was directly observed at room temperature of 25 °C, which included original state, large strain up to 70% state and recovery state. To further evaluate the cyclic resilient property of 3D printed rGO/CNTs@Ni(OH) ₂ and rGO/CNTs@α-Fe ₂ O ₃ microlattices, compression cycling was performed at 60% strain for 10 cycles. Figures 11 and 12 show near-full recoverability after three cycles of compression up to 60% strain for 3D printed rGO/CNTs@Ni(OH) ₂ and rGO/CNTs@a- Fe ₂ O ₃ microlattices respectively, indicating excellent elastometric response.

3D printed quasi-solid-state Ni-Fe battery (QSS-NFB) device

Electrochemical measurements All electrochemical measurements were carried out using a VMP3 electrochemical workstation (Bio-Logic, Claix, France).

For three-electrode system tests, the 3D printed rGO/CNTs@Ni(OH) ₂ cathode or rGO/CNTs@α-Fe ₂ O ₃ anode were directly used as the working electrode, a platinum (Pt) foil served as a counter electrode, a silver/silver chloride (Ag/AgCI) electrode (in saturated potassium chloride (KCI)) was employed as the reference electrode, in which 3.0 M potassium hydroxide (KOH) aqueous solution was used as the electrolyte.

For two-electrode system tests, the 3D printed rGO/CNTs@Ni(OH) ₂ and 3D printed rGO/CNTs@α-Fe ₂ O ₃ were used as the cathode and anode, respectively. To assemble the full cell with aqueous electrolyte, both the electrodes were directly immersed into the beaker filled with 3.0 M KOH electrolyte and tested.

To assemble the full cell with quasi-solid-state gel electrolyte, first a solid-state electrolyte was achieved by dissolving 6.0 g poly(vinyl alcohol) (PVA) into 30 mL H2O and then mixed with 30 mL 6.0 M KOH solution at 85°C by stirring, and then both the electrodes were immersed into the electrolyte and left under ambient conditions for 15 minutes before they were assembled face-to-face, with the polymer electrolyte acting as both the electrolyte and the separator. After the electrolyte was solidified, a quasi- solid-state Ni-Fe battery device with light weight and mechanical robustness can be packed and further tested for its electrochemical performances. To assess the feasibility of 3D printed Ni-Fe batteries, the electrochemical performance of the 3D printed rGO/CNTs-based microlattices electrodes were evaluated with increasing thickness from 1 mm to 4 mm in a three-electrode configuration.

For 3D printed rGO/CNTs@Ni(OH) ₂ cathodes, the mass loadings were 7.4, 15.3, 23.2 and 30.8 mg cm ^-2 (vs. rGO/CNTs microlattices electrodes) at a thickness of 1 mm, 2 mm, 3 mm and 4 mm, respectively. The areal capacity increased proportionally with electrode thickness (Figures 13(a) and 13(b)), with a high areal (or specific) capacity of about 11.64 mAh cm ^-2 (or ~377.8 mAh g ^-1 , based on active material mass) at a 4 mm thickness. At different thickness, all the rGO/CNTs@Ni(OH) ₂ electrodes showed similar specific and volumetric capacities, indicating remarkable ionic diffusion independence (Figures 13(c) and 13(d)).

For 3D printed rGO/CNTs@α-Fe ₂ O ₃ anodes, a similar trend was observed with different thickness, which showed a high areal (or specific) capacity of -16.78 mAh cm- ² (or -486.5 mAh g ^-1 , based on the mass of active material) at 4 mm thickness (Figures 14(a)-(d)). Notably, the pure 3D printed rGO/CNTs substrates contributed negligible capacity to the electrodes, as seen in Figure 15. Compared with conventional manufacturing electrodes (Figures 16(a) and (b)), the 3D- printed interconnected hierarchical pores established unobstructed channels for ionic transport from bottom-up to surface of the 3D printed electrodes. Figure 16(a) shows a comparison of the specific capacity of the 4 mm thickness 3D printed rGO/CNTs@Ni(OH) ₂ electrode with the values of previously reported Nickel-based electrodes: nano-Ni(OH) ₂ /C composite (Lacey et. al. , 2018), β-Ni(OH) ₂ (Tang, Zhou, et. al., 2018), P-NiCo ₂ O ₄ NWAs (Qiu et. al., 2018), Ni(OH) ₂ /NG (Zhu, Han, et. al., 2018), Ni(OH) ₂ /MWNT (Lacey et. al., 2018), GF/CNTs/Ni(OH) ₂ (Jiang et. al., 2018), β-Ni(OH) ₂ /CNFs (Li, Gao, et. al., 2017), CC-CF@NiO (Li, Zhu, et. al., 2017), Ni(OH) ₂ /GS (Li, Li, et. al., 2017). Figure 16(b) shows a comparison of the specific capacity of the 4 mm thickness 3D printed rGO/CNTs@α-Fe ₂ O ₃ anode with the values of previously reported iron-based electrodes: FeOx/Graphene (Lacey et. al., 2018), Fe NPs@C (Zhu, Han, et. al., 2015), GF/CNTs/Fe ₂ O ₃ (Jiang et. al., 2018), CC-CF@Fe ₃ O ₄ (Li, Zhu, et. al., 2017), FeOx/CNTs (Chang et. al., 2018), Graphene/Fe ₃ O ₄ paper (McOwen et. al., 2018), Fe ₂ O ₃ -15 (Yang et. al., 2013), Fe ₃ O ₄ -rGO (Wang et. al., 2017), Fe ₂ O ₃ NTs@CC (Li et. al., 2018), Fe ₂ O ₃ N Ps/Graphene (Wang, Zhang, et. al., 2016).

Moreover, the sufficient junctions of adjacent filaments and close-knitted contacts due to the compact packing order ensured multitude of ionic and electronic pathways. Finally, the excellent compressibility of the free-standing electrode avoided permanent deformation and cracking subjecting to external mechanical stimulation. These contributed to the excellent electrochemical performances of the 3D printing batteries.

Results

Figure 17 shows the electrochemical characterizations, on the left column 3D printed rGO/CNTs@Ni(OH) ₂ microlattices cathode, and on the right column 3D printed rGO/CNTs@α-Fe ₂ O ₃ microlattices anode. Figures 17(a) and (d) show the cyclic voltammetry curves of at different scan rates from 5 to 40 mV/s, and Figures 17(b) and (e) show the galvanostatic charging-discharging curves of at various current densities. Figures 17(c) and (f) show the capacity retention of both 3D printed electrodes tested at a scan rate of 200 mA cm ^-2 for 5000 cycles. Insets of Figures 17(c) and (f) show the corresponding CV curves at the first cycle and the 5000 ^th cycle.

Cyclic voltammograms (CV) curves at scan rates from 5 to 40 mV s ^-1 for the 3D printed rGO/CNTs@Ni(OH) ₂ microlattices electrode (Figure 17(a)) revealed oxidation and reduction reactions corresponding to the reversible conversion of Ni(OH) ₂ to NiOOH, for which the chemical reaction equations can be expressed as follows:

From galvanostatic charging-discharging (GCD) curves shown in Figure 17(b), the sample shows apparent charge and discharge platforms, which matched well with the CV curves. The specific capacity (based on the total mass of Ni(OH) ₂ and rGO/CNTs) of 3D printed rGO/CNTs@Ni(OH) ₂ microlattices electrode were 296.9 mAh g ^-1 at low scan rate of 10 mA cm ^-2 , and the specific capacity retained at 99.3 mAh g ^-1 at a high scan rate of 100 mA cm ^-2 . The CV curves were also performed on the 3D printed rGO/CNTs@α-Fe ₂ O ₃ microlattices electrode from 5 to 40 mV s ^-1 , as shown in Figure 17(d). For the CV curve at 40 mV s ^-1 , the 3D printed rGO/CNTs@α-Fe ₂ O ₃ electrode displayed two oxidization peaks at around -0.85 and -0.44 V in the anodic scan, corresponding to the formation of Fe(OH) ₂ (with the oxidization from Fe ⁰ to Fe ²⁺ ) and FeOOH (with the oxidization from Fe ²⁺ to Fe ³⁺ ), respectively. One well-defined reduction peak at around -1.07V and -1.15 V were found in the cathodic curve, which is assigned to the reduction from Fe ³⁺ to Fe ²⁺ and Fe ²⁺ to Fe ⁰ , respectively. The chemical reaction equations for the charging- discharging process can be expressed as follows:

The GCD tests as shown in Figure 17(e) indicated capacity maintenance of over 391.2 mAh g ^-1 at a low current density of 10 mA cm ^-2 . At a high current density of 100 mA cm- ² , the specific capacity was retained at 186.7 mAh g ^-1 . Moreover, these hybrid electrodes showed good cycle stability with capacity retention at about 91.2% for 3D printed rGO/CNTs@Ni(OH) ₂ microlattices electrode (Figure 17(c)) and about 93.6% for 3D printed rGO/CNTs@α-Fe ₂ O ₃ microlattices electrode (Figure 17(f)) after 5000 cycles at a scan rate of 200 mA cm ^-2 . In addition, both the 4 mm thick electrodes achieved an ultrahigh areal capacity, which is higher than for previously reported electrodes (Figure 18).

Figure 18 shows a plot comparing the areal capacity of 3D printed rGO/CNTs@Ni(OH) ₂ and 3D printed rGO/CNTs@α-Fe ₂ O ₃ electrodes with the values of previously reported electrodes with high mass loading: Mnx-h (Wang, Kong, et. al., 2016), Ni-HAB MOF (Li, Han, et. al., 2018), CNT/V ₂ O ₅ (Zhang et. al., 2017), PPy (Xu et. al., 2016), Graphene (Bin et. al., 2018), MoO ₃ (Jiang et. al., 2019).

To further evaluate the application of the 3D printed rGO/CNTs@Ni(OH) ₂ and rGO/CNTs@α-Fe ₂ O ₃ microlattices electrodes, a 3D printed quasi-solid-state Ni-Fe battery (QSS-NFB) device was assembled with a 6M polyvinyl alcohol (PVA)/KOH gel electrolyte (Figure 20(a)). For comparison, a 3 M KOH aqueous electrolyte was also used for a full aqueous Ni-Fe battery assembly (Figure 19(a)).

As seen from Figure 19(b), the asymmetric device achieved a working voltage window of 2.0 V, larger than the three-electrode device configuration. Figure 19(c) and Figure 20(b) show the CV curves of the Ni-Fe battery in both electrolytes (3 M KOH and 6 M

PVA/KOH) at various scan rates from 5 to 50 mV s ^-1 , respectively. The CV curves of the Ni-Fe battery in both electrolytes are similar with two obvious redox profiles, attributed to the following overall reaction: Additionally, the GCD curves in Figure 19(d) (in 3 M KOH) and Figure 20(c) (in 6 M PVA/KOH) also confirmed the redox reactions. The specific capacities of the 3D printed Ni-Fe batteries at varied current density from 10 to 300 mA cm ^-2 in both electrolytes are shown in Figure 20(d). A high specific capacity of 268.3 mA h g ^-1 was obtained at 10 mA cm ^-2 in 6 M PVA/KOH gel electrolyte, and 116.6 mA h g ^-1 at a high current density of 300 mA cm ^-2 . In comparison, the quasi-solid-state Ni-Fe battery with 6M PVA/KOH gel electrolyte achieved a high capacity of 206.4 mA h g ^-1 at 10 mA cm ^-2 (or 102 mA h g ^-1 at 300 mA cm ^-2) , which is about 76.9% (or about 87.9 %) of that obtained from aqueous electrolyte.

The capacity obtained from aqueous electrolyte is better than gel electrolyte, which is due to the lower resistance that improve electrochemical reaction. As shown in Figure 20(e), the capacity retention in gel electrolyte achieves about 91.3% even after 10,000 cycles, in stark contrast aqueous electrolyte (about 85.6% after 15,000 cycles, as seen in Figure 19(e)), indicating the remarkable performance stability of the 3D printed QSS- NFB devices. Ex situ SEM characterization after cycling test indicates high retainability of the structure, further indicating excellent cycling behaviour.

Effect of different thicknesses

The QSS-NFB devices with different thicknesses (2-8 mm, as QSS-NFB-n, n = 2, 4, 6, or 8 mm respectively) were studied at various current densities (Figure 21(a)). Areal capacities of these devices expanded quasi-linearly with electrode thickness, suggesting scalability of the 3D-printing technology in improving the capacity Ni-Fe battery. Moreover, the increment in cell thickness did not compromise specific and volumetric capacities (Figure 21(b)), further illustrating the feasibility of the 3D printed electrodes for practical applications. Specifically, QSS-NFB-8 device achieved impressive areal capacity of 13.2 mAh cm ^-2 and specific capacity of 206.4 mAh g ^-1 , which was higher than known energy storage devices (Figure 21(c)), such as Ni-Bi battery (Zeng et. al., 2018), 3DP-MWCNT/G//3DP-NCS/G (Tang, Zhu, et. al, 2018), AC/CNT/rGO//AC/CNT/rGO (Tang, Zhu, et. al., 2018), CC-CF@Fe ₃ O ₄ //CC-CF@NiO (Lacey et. al., 2018), CC-CF@ZnO//CC-CF@NiO (Li, Zheng, et. al., 2018), NH4 ⁺ ion battery (Wu et. al., 2017), Ni(OH) ₂ //FeO _x (Wang et. al., 2012), Zn//ZnMn ₂ 0 ₄ (Zhang et. al., 2016), Ni(OH) ₂ //Fe ₂ O ₃ (Liu et. al., 2014), Aqueous Na-ion battery (Dong et. al., 2016).

Figure 21(d) shows the volumetric energy density of the 3D printed QSS-NFB-8 device at various power density (based on the total volume of the device), a high volumetric energy density of 28.1 mWh cm ^-3 at a power density of 10.6 mWW cm ^-3 , and 13.9 mWh cm ^-3 at an exceedingly-high power density of 318.8 mW cm ^-3 were achieved, in stark improvement as reported in the references cited above. Practically proven, a single cell is able to power a small electric fan, and two cells in series can light up a red LED diode (3 mm, 1.8-2.0 V).

Mechanical stress/strain To further evaluate the feasibility of the 3D printed battery for portable/wearable devices, the performance robustness was tested by subjecting the samples to extreme mechanical stress/strain.

Ag films on poly(ethylene terephthalate) (PET) substrates were used as current collectors with 6 M PVA/KOH as electrolyte. In the compressible QSS-NFB device, the solid electrolyte-coated device electrodes retained its highly-porous nature. The QSS- NFB device exhibits similar global compressive deformation rGO/CNTs-based microlattices electrodes (Figure 22).

After a stress-strain cycle, the hysteresis curve almost returned to the origin, suggesting that the compressibility of QSS-NFB device is fully reversible without plastic deformations. Furthermore, the QSS-NFB device retained its initial shape/thickness after extreme compression in various repetitions. Figure 23(a) shows the capacity retention of QSS-NFB device under various strains. At 60% strain, there was about 10% loss in its specific capacity with a marginal 2.14 % loss in its original specific capacity after the structure of the device is restored. Subjecting the quasi-solid-state Ni-Fe battery device at various compressible strain constituted little electrochemical adaptations/response, as indicated by the CV profiles (insets of Figure 23(a)), indicating excellent electrochemical stability, which was consistent to the charge/discharge profiles (Figure 23(b)). Meanwhile, impedance investigation (EIS) was employed to investigate the kinetics of QSS-NFB device under various strains (Figure 23(c)). The Nyquist plots shows a sloped line in the low frequency region which represents the Warburg impedance, corresponding to the diffusion of ions into the cathode material (Zhou et. al. , 2013). The results suggest that the ionic diffusion retained very high-performance robustness under 60% compressive strain under extreme repetitive test (<10 times) (Figure 23(d)). The electrical resistance of the QSS-NFB device exhibited minute decrement over multitude of extreme repetitive trials (inset of Figure 23(d)), further indicating remarkable structural resilience.

To evaluate the rechargeability and stability of the QSS-NFB device under various compression conditions, 500 cycles were performed at each strain were recorded at a high current density of 300 mA cm ^-2 (Figure 23(e)). Relative to the initial capacity, the capacity retention was maintained at 84.5% after suffering from different strains during 2000 cycles. Recovered to its original state, the capacities retention is at about 92.5 % with about 89.1 % after 500 cycles, again implying that the device can effectively buffer and alleviate the mechanical compression/releases.

Multiple QSS-NFB devices To evaluate the suitability of combining several QSS-NFB devices (connected either in series, in parallel, or in combinations of the two) to obtain the required power or capacity (voltage and/or current, respectively), four 3D printed compressible QSS-NFB devices were incorporated into one unit in series by designing the Ag film patterns on PET. The resultant integrated supercapacitor unit was powerful enough to light up a blue LED lamp (3.2 V; D: 5 mm) when fully charged.

Furthermore, the integrated QSS-NFB devices unit worked well when subjected mechanical stresses, which strongly suggests full practical suitability and readiness. The CV curves of four 3D printed compressible QSS-NFB device in series (Figure 24(a)) exhibited an enhanced potential range of 0-8.0 V, which is four times that of a single 3D printed compressible QSS-NFB device. It is also reflected by the charge/discharge curve (Figure 24(b)), where the charge potential is up to 8.0 V, for realistic power requirements.

Conclusion

In conclusion, compressible 3D printed rGO/CNTs-based microlattices electrodes by combining a direct ink writing strategy and solvothermal method has been provided. The 3D printed rGO/CNTs hybrid aerogel microlattices retained its hierarchical architecture and formed 3D continuous skeletons structure under synthesis requirements. Direct synthesis of electrochemically active inorganic nanomaterials (a- Fe ₂ O ₃ and Ni(OH) ₂ ) on 3D printed rGO/CNTs microlattices retained the morphological advantage of the active precursor, providing strong covalent coupling between inorganic nanocrystals and carbon, leading to rapid electron transfer kinetics.

More importantly, the structure of 3D printed rGO/CNTs-based microlattices were reversible after the stress was fully removed without plastic deformations. Based on PVA/KOH gel electrolyte, the highly compressible 3D printed QSS-NFB devices with integrated configuration were prepared by using 3D printed rGO/CNTs@Ni(OH) ₂ microlattices and 3D printed rGO/CNTs@α-Fe ₂ O ₃ microlattices electrodes as cathode and anode, respectively. The distinctive configuration enabled excellent electrochemical performance stability under various compressible strains. The specific capacity of the 3D printed compressible QSS-NFB devices constituted only <10% of maximum capacity under extreme 60% compressible strain in various repetitive trials.

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.