SILICON-NANOGRAPHITE AEROGEL-BASED ANODES FOR BATTERIES

Abstract

The present invention relates to a silicon-nanographite aerogel for use as an anode in a battery, such as a lithium ion battery, comprising a matrix of nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, and silicon nanoparticles having a diameter between 1 nm and 100 nm, whereby the aerogel has a three-dimensional structure with pores between the flakes, whereby the specific surface area accommodates a volume expansion of the silicon nanoparticles of at least 400% during lithiation, and wherein the surfaces of the nanographite flakes are for 10 to 90% covered with nanoparticles of silicon or wherein the aerogel has a specific surface area between 10 and 500 m.sup.2/g as measured using a BET (Braunauer-Emmett-Teller). The invention also relates to a method of making the aerogel and an electrode comprising the aerogel.

Claims

1. A silicon-nanographite aerogel for use as an anode in a battery, such as a lithium ion battery, comprising a matrix of nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, and silicon nanoparticles having a diameter between 1 nm and 100 nm, whereby the aerogel has a three-dimensional structure with pores between the flakes, whereby the specific surface area accommodates a volume expansion of the silicon nanoparticles of at least 400% during lithiation, and wherein the surfaces of the nanographite flakes are for 10 to 90% covered with nanoparticles of silicon.

2. A silicon-nanographite aerogel for use as an anode in a battery, such as a lithium ion battery, comprising a matrix of nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, and silicon nanoparticles having a diameter between 1 nm and 100 nm, whereby the aerogel has a three-dimensional structure with pores between the flakes, whereby the specific surface area accommodates a volume expansion of the silicon nanoparticles of at least 400% during lithiation, and wherein the aerogel has a specific surface area between 10 and 500 m.sup.2/g as measured using a BET (Braunauer-Emmett-Teller).

3. The silicon-nanographite aerogel according to claim 1, wherein the surfaces of the nanographite flakes are for 10 to 90/o covered with nanoparticles of silicon and wherein the aerogel has a specific surface area between 10 and 500 m.sup.2/g as measured using the BET (Braunauer-Emmett-Teller).

4. The silicon-nanographite aerogel according to claim 1, wherein the amount of silicon nanoparticles is at least 3 wt %, wherein wt % are weight percentages of the total weight of the aerogel.

5. The silicon-nanographite aerogel according to claim 1, wherein at least 50% of the surfaces of the nanographite flakes are at least for 10% or 50% covered with nanoparticles of silicon.

6. The silicon-nanographite aerogel according to claim 1, whereby the ratio of nanographite to silicon is about 0.1 to 40:1.

7. The silicon-nanographite aerogel according to claim 1, wherein the nanographite flakes have a thickness between 0.34 nm and 100 nm and a length between 100 nm and 50 μm.

8. An electrode for use as an anode in a battery, such as a lithium ion battery, comprising the silicon-nanographite aerogel according to claim 1, and added untreated nanographite, and a binder, such as sodium alginate.

9. The electrode according to claim 8, whereby the electrode has a specific capacity of 455 mAh/g after 50 to 200 cycles with a coulombic efficiency of 97% at a current density of 100 mA/g.

10. The electrode according to claim 8, whereby an electrical conductivity of the electrode is in the range of 2.5×10.sup.3 kS/m to 10×10.sup.3 kS/m.

11. A method for manufacturing the silicon-nanographite aerogel according to claim 1, comprising the steps of: 1) dispersing the silicon particles in a water-soluble biodegradable, biocompatible, and nontoxic polymer solution, such as polyvinyl alcohol, 2) adding nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, 3) heating the aerogel to at least 500° C., in an inert atmosphere, thereby vaporizing the silicon particles, 4) cooling the aerogel to room temperature, thereby condensing the silicon vapor on the surfaces of the nanographite flakes, whereby the product obtained in step 4) is coated with carbon from vaporized carbon present in the polymer solution used in step 1).

12. The method according to claim 11, wherein the nanographite flakes have a thickness between 0.34 nm and 100 nm and a length between 100 nm and 50 μm.

13. The method according to claim 11, wherein the weight ratio of silicon to nanographite flakes is 0.1 to 40:1.

14. The method according to claim 11, wherein the condensed silicon nanoparticles have a diameter between 1 and 100 nm.

15. The silicon-nanographite aerogel according to claim 1, prepared by 1) dispersing the silicon particles in a water-soluble biodegradable, biocompatible, and nontoxic polymer solution, such as polyvinyl alcohol, 2) adding nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, 3) heating the aerogel to at least 500° C. in an inert atmosphere, thereby vaporizing the silicon particles, 4) cooling the aerogel to room temperature, thereby condensing the silicon vapor on the surfaces of the nanographite flakes, whereby the product obtained in step 4) is coated with carbon from vaporized carbon present in the polymer solution used in step 1).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

[0064] FIG. 1 shows thermogravimetric analysis (TGA) curve of the sample silicon-nanographite aerogel (SNGA) and silicon.

[0065] FIG. 2 shows a) X-ray diffraction pattern and b) Raman spectra of silicon (Si), nanographite (NG)e, and silicon-nanographite aerogel (SNGA).

[0066] FIG. 3 shows an SEM images (a, b and c), TEM image (d) and corresponding SAED pattern (e) of SNGA.

[0067] FIG. 4 shows an SEM images (a and b) of the NG and (c and d) SNGA/NG electrodes at different magnifications.

[0068] FIG. 5 shows (a and b) Cross sectional SEM images of the SNGA/NG electrode, and (c) In beam (SE:BSE) cross-sectional image of the SNGA/NG electrode.

[0069] FIG. 6 shows a) Cyclic voltammograms of the SNGA/NG electrode at a scan rate of 0.1 mV s-1. b) Typical charge-discharge profiles of the SNGA/NG electrode at the 1st (0.09 C), 5th (0.09 C), 25th (0.27 C), 50th (0.45 C), 100th (0.9 C), and 200th (0.09 C) cycles. c) Specific capacities of the SNGA/NG, NG and SNG electrodes at a current density of 100 mA g-1 (equivalent to 0.09 C). d) Coulombic efficiency vs cycle number of the SNGA/NG, NG and SNG electrodes. e) Rate performance of the SNGA/NG, NG and SNG electrodes at different current densities 0.1 A g-1 (0.09 C), 0.3 A g-1 (0.18 C), 0.5 A g-1 (0.45 C), and 1 A g-1 (0.9 C).

[0070] FIG. 7 shows schematically the preparation of silicon-nanographite aerogel.

DETAILED DESCRIPTION

Definitions

[0071] As used herein “powder” means micrometer sized silicon. The size of the silicon powder particles is between 0.5 micrometer and 100 micrometer.

[0072] As used herein “nanoparticles” means silicon particles having a size of between 1 nanometer and 100 nanometer.

[0073] As used herein “nanographite” means graphite consisting of graphene, multilayer graphene, and graphite nanoplatelets, whereby the flakes have a thickness of between 0.34 nanometer and 100 nanometer and a length or lateral size between 100 nanometer and 50 micrometer.

[0074] As used herein “room temperature” means a temperature between 16 and 25° C.

[0075] As used herein “inert atmosphere” means an atmosphere devoid of reactive gases.

[0076] The invention relates to a silicon-nanographite aerogel comprising a matrix of nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, and silicon nanoparticles.

[0077] The silicon nanoparticles may have a diameter between 1 nm to 2 μm, or between 1 nm to 1 μm, or between 1 nm and 500 nm, or between 1 nm and 250 nm, or between 1 nm and 150, or between 1 nm and 100 nm, or between 5 and 100 nm. The silicon nanoparticles are made from silicon microparticles having a diameter between 0.5 μm and 100 μm, or between 0.5 μm and 50 μm, or between 0.5 μm and 25 μm, or between 0.5 μm and 10 μm, or between 0.5 μm and 5 μm, or between 0.5 and 1.5 μm.

[0078] The nanographite flakes may have a thickness between 0.34 nm and 100 nm, or between 0.34 nm and 75 nm, or between 0.34 nm and 60 nm, or between 0.34 nm and 50 nm.

[0079] The nanographite flakes may have a length between 100 nm and 50 μm, or between 100 nm and 25 μm, or between 100 nm and 10 μm.

[0080] The aerogel has a three-dimensional structure with pores between the flakes, whereby the specific surface area accommodates a volume expansion of the silicon nanoparticles of at least 400% during lithiation. The specific surface area of the aerogel can be used as a measure for the porosity of the aerogel. The aerogel may have a specific surface area between 10 and 750 m.sup.2/g as measured using a BET (Braunauer-Emmett-Teller). The specific surface area may be between 10 and 500 m.sup.2/g, or between 10 and 250 m.sup.2/g, or between 10 and 150 m.sup.2/g, or between 10 and 100 m.sup.2/g, or between 10 and 90 m.sup.2/g, or between 20 and 90 m.sup.2/g, or between 30 and 90 m.sup.2/g, or between 40 and 90 m.sup.2/g. The specific surface area may be around 1000 m.sup.2/g or less, or 750 m.sup.2/g or less, or 600 m.sup.2/g or less. In an aspect the specific surface area is 400 m.sup.2/g or less. In another aspect, the specific surface area is 125 m.sup.2/g or less, or 100 m.sup.2/g or less.

[0081] The surfaces of the nanographite flakes are covered with nanoparticles of silicon. The surfaces may be covered for 10 to 90% with nanoparticles of silicon. Some flakes may only be covered for 10 to 30%, while other flakes may be covered for 50 to 99%. At least 50 to 75% of the surfaces of the nanographite flakes may be for 10 to 90%, or 20 to 90%, or 30 to 90%, or at least for 20% or 50% covered with nanoparticles of silicon.

[0082] The surfaces of the nanographite flakes may be for 10 to 90% covered with nanoparticles of silicon and the specific surface area may be between 10 and 500 m.sup.2/g as measured using the BET (Braunauer-Emmett-Teller). Or the surface may be for 30 to 90% covered with nanoparticles of silicon and the specific surface area may be between 30 and 100 m.sup.2/g. Or the surface may be for 50 to 90% covered with nanoparticles of silicon and the specific surface area may be between 50 and 100 m.sup.2/g.

[0083] The amount of silicon nanoparticles in the aerogel may be at least 3 wt %, wherein wt % are weight percentages of the total weight of the aerogel. The amount may be at least 5 wt %, or at least 6 wt %, or between 3 and 100 wt %, or between 4 and 98 wt %, or between 6 and 80 wt %.

[0084] The ratio of nanographite to silicon may about 0.1 to 40:1, or 1:1 to 4:1. The ratio of nanographite to silicon may be about 1 to 4:1, or about 1.5 to 4:1.

[0085] The invention also relates to an electrode for use as an anode in a battery comprising the silicon-nanographite aerogel as defined anywhere above together with and added untreated nanographite, and a binder.

[0086] The electrode may comprise the silicon-nanographite aerogel comprising or consisting of a matrix of nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, and silicon nanoparticles having a diameter between 1 nm and 90 nm, whereby the nanographite flakes have a thickness between 0.344 nm and 90 nm and a length between 100 nm and 25 μm, and the amount of silicon nanoparticles in the aerogel may be at least 5 wt %, and the specific surface area is between 25 and 125 m.sup.2/g.

[0087] The binder may be selected from the group comprising or consisting of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), lithium polyacrylate (Li-PAA), sodium polyacrylate (PAANa), polyacrylic acid (PAA), chitosan, alginate, gelatine, poly(methacrylic acid), polyvinyl alcohol, polyamide, chitosan oligosaccharides, and the like. The binder may be sodium alginate.

[0088] The electrode may have a specific capacity of 455 mAh/g after 50 to 200 cycles, or after 100 to 200 cycles, or after 150 to 200 cycles, or after at least 200 cycles, with a coulombic efficiency of 97% at a current density of 100 mA/g.

[0089] The electrode may have an electrical conductivity of the electrode is in the range of 2.5×10.sup.3 kS/m to 10×10.sup.3 kS/m.

[0090] The invention also relates to a method for manufacturing the silicon-nanographite aerogel as defined anywhere above, comprising or consisting of the steps of:

[0091] 1) dispersing the silicon particles in a water-soluble biodegradable, biocompatible, and nontoxic polymer solution,

[0092] 2) adding nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets,

[0093] 3) heating the aerogel to at least 500° C. in an inert atmosphere, thereby vaporizing the silicon particles,

[0094] 4) cooling the aerogel to room temperature, thereby condensing the silicon vapor on the surfaces of the nanographite flakes,

[0095] whereby the product obtained in step 4) is coated with carbon from vaporized carbon present in the polymer solution used in step 1).

[0096] The polymer solutions may be selected from the group comprising or consisting of polyacrylic acid, sodium alginate, nylon, polyethylene, polyester, Teflon, cellulose. The polymer solutions may be polyvinyl alcohol.

[0097] The silicon microparticles in step 1) may have a diameter between 0.5 μm and 100 μm, or between 0.5 μm and 50 μm, or between 0.5 μm and 25 μm, or between 0.5 μm and 10 μm, or between 0.5 μm and 5 μm.

[0098] The condensed silicon nanoparticles may have a diameter between 1 and 100 nm, or between 5 and 100 nm.

[0099] The method defined above may be used to manufacture a silicon-nanographite aerogel comprising or consisting of a matrix of nanographite flakes consisting of a mixture of graphene, multilayer graphene and graphite nanoplatelets, and silicon nanoparticles having a diameter between 1 nm and 90 nm, whereby the nanographite flakes have a thickness between 0.34 nm and 90 nm and a length between 100 nm and 25 μm, and the amount of silicon nanoparticles in the aerogel may be at least 5 wt %, and the specific surface area is between 25 and 125 m.sup.2/g.

[0100] Experimental

[0101] Materials and Methods

[0102] Polyvinyl alcohol (PVA) (Average molecular weight: 9000-10000) and sodium alginate were purchased from Sigma Aldrich. Nanographite (NG) was produced using a large-scale tube shear process that allows the high volume and low-cost production of nanographite-based devices according to routes described in Blomquist, N. et al. Large-scale production of nanographite by tube-shear exfoliation in water. PLoS One 11, 1-11 (2016), and Blomquist, N. et al. Metal-free supercapacitor with aqueous electrolyte and low-cost carbon materials. Sci. Rep. 7, 39836 (2017)., without further modification.

[0103] Nanographite is produced by hydrodynamic tube-shearing process which can produce large quantities of nanographite. This process produces micrometer-wide and nanometer-thick flakes of nanographite. The exfoliation occurs in an aqueous environment without using any toxic chemicals or organic solvents, making the process environmentally friendly.

[0104] Nanographite is a mixture of graphene, multilayer graphene, and graphite nanoplatelets. The particles are less than 100 nm thick. Silicon powder, with particle size about 1 μm, measured by SEM, was obtained from VestaSi Europe AB. 0.25 g of silicon was dispersed in PVA solution (2 wt %) under vigorous stirring for 30 min using Ultra-Turrax T25 with an S 25 N-10 G shear head at 10 k rpm. Then, 0.5 g of exfoliated nanographite was added to the suspension and stirred for 30 min to form a silicon/nanographite suspension. The suspension was stirred during heating at 90° C. using a magnetic stirrer at 1200 rpm to form silicon-nanographite hydrogels. The hydrogels were washed several times with distilled water and freeze dried at −30° C. for 24 h. Subsequently, the hydrogels were heat in a tube furnace at 800° C. for 2 h in a nitrogen atmosphere to form the silicon-nanographite aerogels (SNGA). A schematic of the preparation of SNGA is shown in FIG. 7.

[0105] Material Characterization

[0106] Structural characterization was conducted using X-ray diffraction (XRD, Bruker D2 phaser) with Cu-Kα (λ=1.54184 Å) radiation in the 2θ range 10° to 80° with the step width 0.01°. Raman spectra of the samples were obtained using a Raman microscope (Horiba XploRA PLUS, laser excitation at 532 nm) in a frequency range of 50-3000 cm.sup.−1. The microstructure of the SNGA and surface morphology of the electrodes was investigated using a field emission scanning electron microscope at 2 kV (FESEM; MAIA3, TESCAN) and a transmission electron microscope at 300 kV (Tecnai F30). Thermogravimetric analysis of SNGA and Silicon was performed using the instrument Mettler Toledo TGA-1 in order to calculate the weight percentage of silicon in the SNGA structure. The sample was heated to 850° C. in nitrogen atmosphere at 20° C./min rate, followed by a 10 min isotherm at 400° C. Afterwards, the measurement was changed to oxygen atmosphere and heated from 400 to 1100° C. at 20° C./min heating rate.

[0107] Electrochemical Measurement

[0108] The SNGA, nanographite, and sodium alginate (as a binder) were mixed at a weight ratio of 60:30:10 using Ultra-Turrax T25 with an S 25 N-10 G shear head at 10 k rpm for 1 h. Sodium alginate was selected as a binder due to its rich content of carboxylic groups, high Young's modulus, and electrochemical stability, which significantly enhances the columbic efficiency, specific capacity, and cycle stability. The mixture of SNGA, NG and binder was deposited on copper foil (1 mg cm.sup.−2) to prepare the electrodes (label: SNGA/NG). Two reference electrodes were prepared by mixing i) nanographite and sodium alginate binder (weight ratio 90:10) labeled as NG and ii) silicon, nanographite and sodium alginate (weight ratio 21:69:10) labeled as SNG. The half cells of these electrodes were assembled in a glove box filled with highly pure argon gas (H.sub.2O<0.1 ppm and O.sub.2<0.1 ppm). Lithium metal foil was used as reference and counter electrode. The electrolyte used was LP40, that is, 1 M Lithium hexafluorophosphate (LiPF.sub.6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a 1:1 weight ratio. Celgard 2325 was used as a separator.

[0109] Cyclic voltammetry (CV) tests were performed between 0.01 and 2.0 V at a scan rate of 0.1 mV s.sup.−1 using a VersaSTAT 4 Potentiostat. Galvanostatic charge-discharge tests of the cells were performed using a LabVIEW-based PXI system in a voltage range between 0 and 1.5 V at various current densities. The capacities and current densities were calculated based on the weight of the active materials (total weight of SNGA+NG in case of SNGA/NG electrode, weight of NG in case of NG electrode and total weight of Si+NG in case of SNG electrode) without binder. All of the electrochemical measurements were conducted at room temperature.

[0110] Results and Discussion

[0111] TGA measurement of the SNGA and Silicon was performed by heating/cooling in nitrogen atmosphere 30-850-400° C. with at the rate of 20° C./min. followed by 10 min. isotherm at 400° C. and is shown in FIG. 1. Thereafter, a switch to oxygen atmosphere was made, followed by heating from 400 to 1100° C. with the 20° C./min. heating rate. TGA measurement of the silicon was carried out to determine the exact percentage of the silicon in the aerogel composite. TGA measurement of silicon shows small gain in weight indicating oxidation of silicon with the formation of SiO.sub.x. From the TGA curve of the SNGA sample, it is observed that the there is no appreciable weight loss in the nitrogen atmosphere. In the oxygen atmosphere, there is a weight loss of 64% started at 400° C. and ends at 800° C., corresponds to combustion of graphite. Based on this result, the content of silicon in the SNGA structure is calculated to be 34.65%. The SNGA, nanographite, and sodium alginate binder were mixed at a weight ratio of 60:30:10 to prepare the electrode. Hence, percentage of silicon is 20.79% in the final electrode.

[0112] The XRD patterns of Si, NG, and SNGA are shown in FIG. 2a. For Si, five diffraction peaks are observed at 28.3°, 47.0°, 55.8°, 68.4°, and 75.5°. These diffraction peaks are related to Bragg's reflections from the (111), (220), (311), (400), and (331) planes of the Si phase (JCPDS no. 27-1402), respectively. For NG, a diffraction peak is observed at 26.1 that corresponds to the (002) plane of graphite. The significant reduction of the characteristic peak of silicon and nanographite observed in the XRD pattern of SNGA may be due to the presence of amorphous substances covering the structure. The thickness and uniformity of amorphous substances blocks the X-rays from reaching the crystalline silicon particles and nanographite flakes suppressing their peaks in corresponding XRD pattern.

[0113] Raman spectroscopy was carried out to study the structural aspects of Si, NG and SNGA samples. FIG. 2b shows the Raman spectra of Si, NG, and SNGA. Both Si and SNGA shows the peak centered at 523 cm.sup.−1 which corresponds to crystalline Si. Thus, it is clear that there is no phase change of Si during the aerogel preparation process. The peaks observed at 1346, 1579, and 2720 cm.sup.−1 of the samples NG and SNGA are related to graphite. These peaks correspond to the D, G, and 2D bands, respectively. Similarly, there is no change in graphite phase.

[0114] SEM images of the SNGA structure are shown in the FIGS. 3a, b and c revealing the presence of nanoparticles on the nanographite flakes. To further analyze these nanoparticles; TEM images of the SNGA structure were taken. FIG. 3d shows the TEM image of the SNGA. TEM analysis further reveals, in SNGA structure the nanoparticles covering the nanographite flakes are of silicon/silica; Both SEM EDS and TEM EDS shows presence of silicon and oxygen, however in SEM EDS oxygen is at higher ratio indicating silicon to be of SiO.sub.x form. The selected area electron diffraction (SAED) pattern shown in FIG. 3e exhibits the rings made up of discrete spots. The rings correspond to reflections with d spacing 0.346 nm, 0.213 nm, 0.1924 nm, 0.121 nm and 0.105 nm. The reflections with d spacing 0.346 nm, 0.213 nm, 0.121 nm and 0.105 nm corresponds to (002), (100), (110) and (201) planes of graphite respectively while the reflection with d spacing 0.1924 nm is due to (220) plane of silicon implying presence of nanographite and silicon. One likely mechanism describing the formation of particles can be understood by a silane route. PVA undergoes pyrolysis at about 230° C., decomposes rapidly and further reacts with silicon in nitrogen atmosphere forming a gaseous silane (SiH.sub.4). The silane gas diffuses over the graphite flakes and further undergoes thermal degradation due to high temperatures forming the silicon nanoparticles over the nanographite flakes (as seen in the SEM and TEM image). However, after aerogel preparation process, part of the silicon nanoparticles undergo oxidation when air comes inside the tube furnace forming SiO.sub.x. Further, the NG aerogel was prepared by the same procedure described above. These structures do not show presence of any nanoparticles on the nanographite flakes. From SEM image, the estimated amount of silicon nanoparticles in the SNGA structure is around 10.37(2) weight %. (Hence, the amount of silicon nanoparticles in the SNGA/NG electrode is 6.2(2) weight %. Therefore, in the SNGA/NG electrode out of 20.79 silicon, 6.2 is nanosized silicon and remaining 14.59 is silicon microparticles.

[0115] FIG. 5 shows (a and b) is a cross sectional SEM images of the SNGA/NG electrode, and (c) In beam secondary electron: back scattered electron (SE:BSE) image cross-sectional image of the SNGA/NG electrode. From FIG. 5 a, b and c it is observed that the SNGA/NG electrode have a porous structure with smaller and larger pores. SE:BSE gives the insights of the electrode wherein the BSE contrast image (FIG. 5c) shows mainly silicon powder remained in the structure which is not converted to the silicon nanoparticles.

[0116] Electrode Analysis

[0117] SEM images of the NG and SNGA/NG electrodes at 20 μm and 100 μm are shown in FIG. 4a, b, c, and d, respectively. From FIGS. 4a and b, the NG electrode contains nanographite flakes of different sizes that are stacked over each other. A few pores are observed in these structures, which are formed due to the stacking of nanographite flakes of different sizes. However, FIGS. 4c and d demonstrate that the SNGA/NG electrode that contains Si-nanographite aerogels with nanographite shows a large number of small and large pores with relatively smaller nanographite flakes.

[0118] The charging/discharging mechanism of the SNGA/NG electrode (as an anode) for lithium-ion batteries was investigated using CV. The CV measurements were performed on half cells in a voltage range between 0.01 to 2.0 V at a scan rate of 0.1 mV s.sup.−1 for five cycles as shown in FIG. 6a. A cathodic peak is observed from 0.78 to 0.46 V in the first scan, which is attributed to the formation of a thin SEO layer. This peak disappeared in the successive cycles, indicating the formation of an SEI in the first cycle. Another cathodic peak is observed in the first cycle at 0.01 V, which is characteristic of lithiation of crystalline and amorphous Si. This splits into two peaks at 0.16 and 0.01 V in the second cycle and becomes sharper in the further cycles. These peaks belong to the lithiation of Si and formation of a Li—Si alloy. In the first cycle, one anodic peak at 0.29 V is characteristic of amorphous Si and splits into two peaks at 0.29 and 0.49 V in the second cycle and becomes sharper in the further cycles. These peaks correspond to the phase transition from Li—Si alloy to Si. After the first cycle, the intensities of the cathodic and anodic peaks increase, indicating improvement in the Li insertion and extraction kinetics.

[0119] To study the electrochemical performance of the SNGA/NG electrode, galvanostatic charge-discharge measurements were conducted at a current density of 100 mA g.sup.−1 (equivalent to 0.09 C) in a voltage range of 0 to 1.5 V. Galvanostatic charge-discharge measurements of the NG and SNG electrodes were also conducted at the same current density and voltage range for comparison. The typical charge-discharge profiles of the SNGA/NG electrode at the 1.sup.st, 5.sup.th, 25.sup.th, 50.sup.th, 100.sup.th, and 200.sup.th cycles is shown in FIG. 5b. The current density in the 1.sup.st, 5.sup.th, 25.sup.th cycles was 0.1 A g.sup.−1 and that of the 50.sup.th, 100.sup.th, and 200.sup.th cycles was 0.3 A g.sup.−1 (0.27 C), 1 A g.sup.−1 (0.9 C), and 0.1 A g.sup.−1, respectively. The first discharge shows two slopes between 0.78-0.46 V and 0.16-0.01 V, which can be correlated with the cathodic peaks observed at the same position in the discharge of the first cycle in CV. This is largely responsible for the electrode's capacity. The slope between 0.78 and 0.46 V (corresponding to the formation of the stable SEI layer) disappears in subsequent cycles while a slope between 0.16 and 0.01V is present in the discharge of subsequent cycles. From the charge-discharge profiles of the SNGA/NG electrode, the plateaus between 0.16-0.01 V are present up to 200 cycles, indicating that lithiation-dilithiation occurs in the Si particles without pulverization and disintegration from the current collector.

[0120] FIGS. 6c and d show the cycling performance and corresponding coulombic efficiency of the SNGA/NG, NG and SNG electrodes at a current density of 100 mA g.sup.−1. In the first cycle, the SNGA/NG electrode has a discharge capacity of 1050 mAh g.sup.−1 and a charge capacity of 1072.2 mAh g.sup.−1 with a coulombic efficiency of 97.9%, while the NG electrode has a discharge capacity of 325.8 mAh g.sup.−1 and a charging capacity of 318.7 mAh g.sup.−1 with a coulombic efficiency of 102.2% and the SNG electrode has a discharge capacity of 890.7 mAh g.sup.−1 and a charging capacity of 870.6 mAh g.sup.−1 with a coulombic efficiency of 102.3%. In the second cycle, the discharge capacity decreases to 944.4 mAh g.sup.−1, 319.8 mAh g.sup.−1 and 577.4 mAh g.sup.−1 for SNGA/NG, NG and SNG, respectively. Finally, in the 30.sup.th cycle, the discharge capacity decreases to 603.1 mAh g.sup.−1, 310 mAh g.sup.−1 and 293.7 mAh g.sup.−1 for SNGA/NG, NG and SNG, respectively. The specific capacity of the SNGA/NG electrode is higher than the NG and SNG electrodes in all the cycles.

[0121] The electrodes SNGA/NG and SNG has the same percentage of silicon but there is a significant difference in their capacities and capacity retention.

[0122] The charge-discharge studies of the SNGA/NG, NG and SNG electrodes were conducted at different current densities of 0.1 A g.sup.−1 (0.09 C), 0.3 A g.sup.−1 (0.27 C), 0.5 A g.sup.−1 (0.45 C), and 1 A g.sup.−1 (0.9 C) and the corresponding results are provided in FIG. 6e. The specific capacities of the SNGA/NG, NG and SNG electrodes in the 25.sup.th cycle are 622.5 mAh g-1, 310.2 mAh g.sup.−1 and 304.8 mAh g.sup.−1 respectively, at a current density of 0.1 A g.sup.−1. After increasing the applied current density to 0.3 A g.sup.−1 (after 25 cycles), the capacity falls by 17% in the SNGA/NG electrode, 7% in the NG electrode and 41% in the SNG electrode, remaining almost constant afterwards. When the applied current density increases to 0.5 A g.sup.−1 (after 50 cycles), there is a decrease in the specific capacity of 13%, 7% and 30% in the SNGA/NG, NG and SNG electrodes, respectively, remaining constant thereafter. Whereas at higher applied current densities (1 A g.sup.−1), there is a 22%, 20% and 49% decrease in the specific capacity in the SNGA/NG, NG and SNG electrodes, respectively, which remains constant. At 100.sup.th cycles, the capacity of the SNGA/NG electrode is significantly larger than that of the NG and SNG electrodes. Further, cyclic stability measurements were conducted on the SNGA/NG electrode at an applied current density of 1 A g.sup.−1 up to 189 cycles and continued up to 200 cycles at 0.1 A g.sup.−1. The corresponding results are shown in FIG. 6e. When the applied current density decreased to 0.1 A g.sup.−1 (after 189 cycles), 74% specific capacity is recovered, and stable capacity is delivered for the SNGA/NG electrode. This indicates that Si continues to contribute to the specific capacity of the electrode up to 200 cycles.

[0123] The specific capacity of the SNGA/NG electrode for the first cycle is 1050 mAh g.sup.−1 (that is almost equivalent to the theoretical capacity 1084 mAh g.sup.−1 calculated based on the weight of the silicon and nanographite) which decreases to 603.1 mAh g.sup.−1 after 30 cycles measured at the current density of 0.1 A g.sup.−1. The capacity retention of SNGA/NG electrode is 57% for the 30.sup.th cycle. Comparing it with previous studies of pristine silicon, milled silicon and heat-treated silicon that show capacity retention of 33%, 32% and 52% respectively for 30.sup.th cycle measured at the current density of 0.1 A g.sup.−1. Further for 100th cycle, the capacity retention for SNGA/NG is 52% while for pristine silicon, milled silicon and heat-treated silicon the capacity retention is 17%, 31% and 19% respectively.

[0124] Jiang et. al. (Free-Standing Si/Graphene Paper Using Si Nanoparticles Synthesized by Acid-Etching Al—Si Alloy Powder for High-Stability Li-Ion Battery Anodes. Electrochim. Acta 188, 777-784 (2016)) have synthesized Si nanoparticles (size ˜ 10 nm) via acid-etching Al—Si alloy powder and further prepared the Si/GO paper followed by thermal reduction at 700° C. The electrochemical study reveals, the said electrode has capacity fade of 47% from initial capacity (3200 mAh g.sup.−1 to 1500 mAh g.sup.−1 for 3.sup.rd cycle). The capacity was calculated based on the weight of active silicon particles only. Based on these calculations, the SNGA/NG electrode, has capacity of 2365 mAh g.sup.−1 after 100 cycles which is 58% higher than this report..sup.33 Lyu et. al. has prepared silicon-based gel in which Si core is covered with phytic acid shell layers through a facile high-energy ball milling method. The capacity was calculated based on the weight of active silicon particles. The gel-based electrode has the capacity of 1300 mAh g.sup.−1 at 0.42 A g.sup.−1 while SNGA/NG electrode has shown the capacity of 1915 mAh g.sup.−1 at 0.5 A g.sup.−1 after 50.sup.th cycle. which is 47% higher than gel electrode.

[0125] In comparison with the above results, SNGA/NG electrode prepared using micron sized silicon particles have shown much better specific capacity and capacity retention. SNGA is prepared by simple, scalable, and cost-efficient method as compared with the methods explained above. The specific energy of SNGA/NG electrode was found to be 787 Wh kg.sup.−1 for the first cycle and for 200.sup.th cycle, the energy density was 341.25 Wh kg.sup.4 which is significantly higher than commercial automotive batteries based on the Si anodes.

TABLE-US-00001 TABLE 1 Comparison of specific capacities based on expected and actual weight percentage of nanosilicon and nanographite Contribution Expected in the Actual contribution specific Specific Contribution from capacity capacity from nanosilicon based on of the nanographite Expected in the Actual the actual No of SNGA/NG in specific weight of specific weight of weight of cycles electrode capacity nanosilicon capacity nanosilicon nanosilicon  30 603.1 mAh g.sup.−1 238.7 mAh g.sup.−1 10.41% 364.4 mAh g.sup.−1 6.2% 217 mAh g.sup.−1 200   455 mAh g.sup.−1 238.7 mAh g.sup.−1   6.2%   217 mAh g.sup.−1 6.2% 217 mAh g.sup.−1

[0126] From FIG. 6c, it is observed that for 30.sup.th cycle, the specific capacity of SNGA/NG and NG electrode is 603.1 mAh g.sup.−1 and 310 mAh g.sup.−1 respectively. Considering the weight percentage of nanographite in the SNGA/NG electrode, it contributes 238.7 mAh g.sup.−1 in the total specific capacity of electrode while the expected contribution from nanosilicon in specific capacity is 364.4 mAh g.sup.−1 implying the weight of nanosilicon in SNGA/NG electrode to be 10.41% (indicated in Table 1). The actual weight of nanosilicon is 6.2% while the expected value of 10.41% falls within upper bound of geometric standard deviation in weight of nanosilicon i.e. 12.4%. However, the decaying of capacity curve indicates this effect to come from the fracturing of the microparticles in the powder.

[0127] Analyzing further for 200.sup.th cycle, the specific capacity of SNGA electrode is 455 mAh g.sup.−1, the expected contribution from nanosilicon in specific capacity is 218 mAh g.sup.−1, which corresponds to 6.2% of weight percent of nanosilicon in the final electrode. The theoretical value of the weight percentage of nanosilicon (6.2%) matches closely with the actual calculated value of 6.2%. This implies that the specific capacity is mainly due to silicon nanoparticles.

CONCLUSIONS

[0128] A simple, scalable, and cost-efficient method for the manufacturing of silicon nanoparticles attached to the nanographite flakes was studied and reported. The presence of silicon/silica nanoparticles attached to the nanographite flakes was confirmed from the SEM and TEM studies. The electrochemical measurements prove a stable specific capacity even at high current rates and good cyclic stability for aerogel-based electrodes. The electrodes show a specific capacity of 455 mAh g.sup.−1 for 200.sup.th cycles with a coulombic efficiency of 97% at a current density of 100 mA g.sup.−1. This performance is fully explained by the contribution of nanosilicon in the electrode. This electrode is made from a scalable and low-cost aerogel method. Electrode performance for 200.sup.th cycle i.e. 455 mAh g.sup.−1 corresponds to 341 Wh kg.sup.−1 which is higher value than currently reported best value of 260 Wh kg.sup.−1 in literature..sup.41 Measured nanosilicon weight was 6.2% and matches closely to the expected weight calculated from electrochemical performance of 6.2%.