WO2022246541A1 - Graphene composite battery electrode particles with void space and methods of making same - Google Patents

Abstract

Composite particles for use as a battery electrode precursor material, and methods for manufacturing such particles. The particles have a lithium-permeable graphene-based external layer defining an inner volume, the inner volume containing a plurality of electrochemically active particles and electrochemically inactive particles, the electrochemically inactive particles capable of volume reduction to form void space within the inner volume. The electrochemically inactive particles may be partially or fully thermally decomposable to form the void space, or a material like a responsive gel susceptible to shrinkage under certain conditions to form the void space. The composite particles can be manufactured by forming a dispersion of graphene and the electrochemically active particles, combining the electrochemically inactive particles with the dispersion, forming the dispersion into particles, and treating the particles to reduce the electrochemically inactive particle volume to form the void space.

Description

GRAPHENE COMPOSITE BATTERY ELECTRODE PARTICLES WITH VOID SPACE AND

METHODS OF MAKING SAME

TECHNICAL FIELD OF THE INVENTION

The present invention relates to battery electrodes, and more particularly to the formation of precursor materials for use in electrode manufacture.

BACKGROUND OF THE INVENTION

The rapid development of electric devices in various practical applications (e.g., electric vehicles and portable electronics) has fueled the demand for energy storage with higher energy density and lower cost/performance ratio. The similarly low redox potential of lithium-silicon alloys to those of the lithiated graphite used in current Li-ion batteries, silicon’s natural abundance and nearly tenfold higher theoretical capacity (3590 mAh/g for LhsSL, at room temperature) compared to 372 mAh/g for UC₆ make silicon one of the most promising anodes for next-generation lithium-ion batteries.

Lithium-ion batteries are one of the most common energy storage devices found in portable electronics, smart phones, and electric vehicles. Improving this technology’s energy density has been key towards enabling these technologies. However, the use of traditional graphite anodes limits further enhancement due to its low theoretical capacity (-372 mAh/g). Therefore, developing new anode materials is important in order to obtain a breakthrough in energy density. Silicon-based anodes have attracted extensive attention due to varying advantages: 1) high theoretical capacity (3590 mAh/g at room temperature, LhsSL), 2) low discharge potential (about 0.2 V with respect to Li/Li⁺) and 3) abundant reserves in the earth.

Before silicon can feasibly replace graphite there are two critical challenges which must be addressed. First, silicon experiences significant volume expansion/shrinkage (> 300%) during lithiation/delithiation. This failing generates substantial internal stresses resulting in the contact loss of bulk electrode and self-pulverization of silicon particles. Furthermore, the repeated volume change leads to the continuous formation of the solid electrolyte interface layer (SEI) which continuously consumes electrolyte. The resulting unstable electrode system during cycling leads to a rapid capacity drop and low coulombic efficiency. Second, silicon has inherently low electric and ionic conductivity leading to inferior rate capability. A common and effective method to overcome these issues is nanostructuring of silicon by tailoring the shape, controlling the dimension, or introducing pores. The resulting porous silicon, silicon nanowires, silicon nanotubes and silicon nanoparticles have much improved damage tolerance and structural stability. Meanwhile, more surface of silicon is exposed which can effectively release the stress generated during cycling, and the resulting shortened distance provides a faster pathway for lithium ions and electrons transportation to accommodate a rapid charge/discharge. However, the nanostructuring cannot limit the continuous consumption of electrolyte due to the unstable SEI, and the electric/ionic conductivity of silicon is still unsatisfactory compared to other anode materials, such as graphite.

Various methods for coating the silicon with carbon-based materials have been developed to further improve the electrochemical performance and maintain a high capacity. A typical way to achieve this target is to coat a layer of polymer at the surface of silicon via in-situ polymerization, followed by thermal carbonization to form a uniform and conformal coating of carbon to wrap around silicon. Although this method can significantly optimize the conductive network and limit the direct contact of silicon with electrolyte to avoid the repeated formation of SEI, the carbon layer still cannot tolerate the substantial volume change in silicon core for long term cycling. Thus, these works based on simple direct carbon coating still display poor cyclic stability, which usually drops to below 80% of the initial capacity after only ~50 cycles. Hence, research has focused on generating void space between hard carbon and silicon to further boost its cyclic stability. These strategies usually take advantage of the removal of sacrificial materials, which do not have direct benefits to electrochemical performance, via the implementation of harsh steps (e.g., HF-etching or high temperature and pressure treatment) leading to production of waste and increase of the overall fabrication cost.

SUMMARY OF THE INVENTION

The present invention is directed to a composite electrode structure where a protective graphene oxide (GO) shell is wrapped around clusters of active material and a method to introduce void space within the core of such structures using a third component which is mixed with the active material and used to control void space within the core. This allows the active material to expand/contract during charge/discharge without causing a significant strain on the graphene shell. This significantly improves the cycle-life of thick electrodes such as silicon anodes. In preferred embodiments of the present invention, a polymer material is used as the third component which can either be wholly or partially consumed/decomposed or subjected to conditions that otherwise reduce initial volume to introduce void space to accommodate the volume changes of the active material during charge/discharge.

In one exemplary embodiment set forth below in detail, a void-space is created within the composite material’s core by incorporating varying amounts of similarly sized polystyrene (PS) nanospheres in a spray dryer feed mixture. The PS may completely decompose during thermal reduction of the graphene oxide shell (forming a reduced graphene oxide (rGO)) and results in Si cores of varying porosity, although some other polymers useful with embodiments of the present invention may only be partially decomposed resulting in a soot-like residue that is a conductive carbonaceous material or nitrogen-doped carbonaceous material which can facilitate conduction. Desirable performance is achieved at a 1:1 volume ratio (PS/Si) leading to capacities of 1638 mAh/(g si_+rco), 1468 mAh/(g si_+rco), and 1179 mAh/(g si_+rco) at 0.1 A/g, 1 A/g, and 4 A/g, respectively. Moreover, at 1 A/g, the capacity retention was found to be 80.6% after 200 cycles during experimentation. At a practical active material loading of 2.4 mg/cm², the electrodes could achieve an areal capacity of 2.26 mAh/cm² at 1A/g.

In another exemplary embodiment set forth below in detail, a poly (ethylene oxide) - carboxymethyl cellulose hydrogel is introduced into the core of crumpled reduced graphene oxide (CrGO) encapsulated silicon nanoparticles. By taking advantage of the volume swelling/shrinkage of hydrogel in wet/dry state, void space can be introduced between the Si-Si and Si-rGO interfaces within the encapsulated structure to buffer the volume change during charge/discharge. The resulting composite displays desirable cyclic stability, retaining ~ 81.7% of its initial capacity (1055 mAh/(g _rco₊c_ei+si)) after 320 cycles at 1 A/g with an active material loading of 1 mg/cm². Even at an increased mass loading (2.5 mg/cm²), the areal capacity of this material only dropped from 2.04 mAh/cm² to 1.61 mAh/cm² with a capacity retention of 79% after 200 cycles at 1 A/g.

According to a first broad aspect of the present invention, there is provided a composite particle for use as a battery electrode precursor material, comprising a lithium-permeable graphene- based external layer defining an inner volume, the inner volume containing a plurality of electrochemically active particles and electrochemically inactive particles, the electrochemically inactive particles capable of volume reduction to form void space within the inner volume. In some embodiments of the first aspect, the electrochemically active particles are selected from the group consisting of silicon, silicon oxide, tin, germanium, antimony, TiO, ZnO, SnO, COO, FeO, MnO, NiO, MoO, MoC, CuO, CU2O, Ce0₂, RuO, carbon, a bimetallic material, a multi-metallic material, an oxide material, a sulfide material, and combinations thereof.

The electrochemically inactive particles may comprise at least partially thermally decomposable material selected from the group consisting of polystyrene, polyethylene glycol (PEG), NaHC0₃, NH4HCO₃, polymethyl methacrylate (PMMA), sucrose, wheat particles, starch, saw dust, chitosan, polyacrylonitrile, and gum Arabic. The electrochemically inactive particles may comprise polystyrene nanoparticles. Alternatively, the electrochemically inactive particles may comprise a responsive hydrogel susceptible to volume decrease such that the responsive hydrogel shrinks to form the void space, and in some embodiments the responsive hydrogel is a carboxymethyl cellulose - poly (ethylene oxide) hydrogel.

The battery electrode is preferably a silicon anode. The electrochemically active particles preferably comprise silicon nanoparticles which expand contract during charge and discharge. In such cases, the void space may allow the silicon nanoparticles to expand into the void space to reduce stress on the graphene-based external layer.

The electrochemically active particles are preferably of similar size to the electrochemically inactive particles.

In some exemplary embodiments the graphene-based external layer comprises graphene oxide nanoparticles. In some such embodiments the electrochemically active particles are of similar size to the electrochemically inactive particles, and the graphene oxide nanoparticles are larger than the electrochemically active particles and the electrochemically inactive particles.

According to a second broad aspect of the present invention, there is provided a method for manufacturing a battery electrode precursor material, comprising the steps of: a. forming a dispersion of graphene and electrochemically active particles; b. combining electrochemically inactive particles capable of volume reduction with the dispersion; c. forming the dispersion into particles with the graphene as an external layer; and d. treating the particles to cause the electrochemically inactive particles to reduce in volume to form void space within the particles.

As noted above, the volume reduction may be due to decomposition (whole or partial) or shrinkage due to drying.

In some exemplary embodiments of the second broad aspect, the step of forming the dispersion of the graphene and the electrochemically active particles comprises mixing the graphene and the electrochemically active particles in a water/ethanol mixture.

The electrochemically active particles are preferably selected from the group consisting of silicon, silicon oxide, tin, germanium, antimony, TiO, ZnO, SnO, COO, FeO, MnO, NiO, MoO, MoC, CuO, Cu₂0, Ce0₂, RuO, carbon, a bimetallic material, a multi-metallic material, an oxide material, a sulfide material, and combinations thereof.

The electrochemically inactive particles may comprise at least partially thermally decomposable material selected from the group consisting of polystyrene, PEG, NaHC0₃, NH4HCO3, PMMA, sucrose, wheat particles, starch, saw dust, chitosan, polyacrylonitrile, and gum Arabic. Alternatively, the electrochemically inactive particles may comprise a responsive hydrogel susceptible to volume decrease such that the responsive hydrogel shrinks to form the void space, which responsive hydrogel may in some embodiments be a carboxymethyl cellulose - poly (ethylene oxide) hydrogel formed by cross-linking of the carboxymethyl cellulose with the poly (ethylene oxide) by citric acid as a cross-linker in the presence of heat.

In some exemplary embodiments the step of forming the dispersion into particles comprises spray-drying the dispersion after combining the electrochemically inactive particles.

Where the electrochemically inactive particles are thermally decomposable to form the void space, the step of treating the particles preferably comprises applying heat to the particles to at least partially decompose the electrochemically inactive particles to form the void space. In embodiments where the graphene is graphene oxide, the step of applying heat may further reduce the graphene oxide to reduced graphene oxide.

Where the electrochemically inactive particles comprise a responsive hydrogel, some exemplary embodiments may further comprise the step after the step of forming the dispersion into the particles of hydrating the particles with water to swell the responsive hydrogel in the particles. The step of treating the particles may then comprise evaporating at least some of the water to shrink the responsive hydrogel to form the void space.

Some exemplary methods may further comprise selecting a ratio of electrochemically active particles to electrochemically inactive particles to selectively tune the amount of the void space that will be formed by the step of treating the particles.

A detailed description of exemplary embodiments of the present invention is given in the following. It is to be understood, however, that the invention is not to be construed as being limited to these embodiments. The exemplary embodiments are directed to particular applications of the present invention, while it will be clear to those skilled in the art that the present invention has applicability beyond the exemplary embodiments set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate exemplary embodiments of the present invention:

FIG. 1 is a schematic illustration of one exemplary embodiment showing the stages of forming a void-filled particle.

FIG. 2 is a schematic illustration of an exemplary drying process.

FIG. 3 shows nanoparticle characteristics.

FIG. 4 shows relative size and thermal stability of precursor and composite materials.

FIG. 5 is an illustration characterizing the void space created by removing polystyrene.

FIG. 6 shows an X-ray photoelectron spectroscopy (XPS) survey of (a) varying samples and (b) pure silicon nanoparticles.

FIG. 7 shows X-ray characterization of VCrGO@Si: high-resolution XPS narrow scan spectra of CGO@Si-1 (a), VCrGO@Si-1 (b) and pure PS (c) for their C 1s peaks, (d) X-ray diffraction (XRD) patterns of varying samples. FIG. 8 illustrates electrochemical performance of various samples.

FIG. 9 is a schematic illustration of an improved conductive network.

FIG. 10 illustrates cyclic stability of various samples.

FIG. 11 illustrates thermogravimetric analysis (TGA) result of VCrGO@Si-1.

FIG. 12 illustrates analysis of the capacity contribution of VCrGO@Si-1.

FIG. 13 is Nyquist plots of VCrGO@Si-0 after the 1st cycle and the 200th cycle.

FIG. 14 is scanning electron microscope (SEM) images of VCrGO@Si-1 after 200 cycles at various magnifications.

FIG. 15 is areal capacity retention of recent technical works reporting Si/C based anode.

FIG. 16 is a comparison of rate performance for various Si/C based anodes.

FIG. 17 is a schematic illustration of an exemplary synthesis process of rGO/gel/Si.

FIG. 18 is a SEM image of spray dried pure gel/Si.

FIG. 19 is an illustration characterizing hydrogel cross-linking and swelling/shrinkage.

FIG. 20 is images of non-cross-linked carboxymethyl cellulose/poly(ethylene oxide)

(CMC/PEO) hydrogel.

FIG. 21 is SEM images of GO/Si before (a) and after (b), GO/gel/Si before (c) and after (d) immersed in water for 12h.

FIG. 22 is (a) an image of rGO/gel/Si (40%), where Si came out and settled down to the bottom during centrifugation washing, (b) and (c) SEM images of resulting material from (a) where crumpled rGO framework is broken.

FIG. 23 is a high-resolution characterization of rGO/Gel/Si.

FIG. 24 is electron energy loss spectroscopy (EELS) elemental mapping for C (a), Na (b), O (c) and Si (d) respectively. FIG. 25 illustrates characterization of various samples.

FIG. 26 is SEM images of the single rGO/gel/Si ball which is conducted for depth profiling measurement.

FIG. 27 illustrates electrochemical performance of various samples.

FIG. 28 illustrates electrochemical performance of various samples.

FIG. 29 illustrates areal capacity retention of various published Si/C based anodes.

FIG. 30 is a TEM image of GO/gel/Si reduced by hydrazine vapor without being immersed in water.

FIG. 31 is Nyquist plots of rGO/gel/Si before and after 320 cycles at 1 A/g.

Exemplary embodiments will now be described with reference to the accompanying drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Throughout the following description, specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. The following description of examples of the invention is not intended to be exhaustive or to limit the invention to the precise form of any exemplary embodiment. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

The present invention is directed to a composite particle for use as a battery electrode precursor wherein a protective graphene oxide (GO) shell is wrapped around clusters of active material and void space is created within the core of such particles using a third component which is mixed with the active material and used to control void space within the core. This allows the active material to expand/contract during charge/discharge without causing a significant strain on the graphene shell, significantly improving the cycle-life of thick electrodes such as silicon anodes.

Exemplary Embodiment Using Polystyrene as Electrochemically Inactive Particles

In a first exemplary embodiment of the present invention, void space is controlled in the Si nanoparticle (NP) core of the crumpled rGO shell using a one-step encapsulation approach in which polystyrene (PS) is wholly or partially decomposed to create the void space. As shown in FIG. 1, a schematic illustration is provided of a synthesis process of voided CrGO encapsulated Si NPs (VCrGO@Si-X, where X is the ratio of PS/Si), where a spray drying process (in the box) can be divided into diffusion/convection segregation process (® to ©) and the capillary collapse process ( ® to (D). Polystyrene nanoparticles (PS NPs) of similar size to the Si NPs are spray dried together to obtain a PS/Si composite encapsulated with crumpled GO. See FIG. 2 for a schematic of an exemplary spray drying system (®N2 flow in ©Spray nozzle ©Drying chamber ©Cyclone ©Product vessel ©Filter). Thermal decomposition of the

PS and reduction of the GO lead to a composite with void fraction and porosity which is tunable by the PS to Si ratio. The impact of introducing void space in this way on the electrochemical performance of the composite structure was used to determine the optimal PS/Si ratio of 1 : 1 which displays both the best rate capability and cyclic stability. These electrodes exhibited a high capacity of 1183 mAh/(g si+rco) after 200 cycles at 1 A/g with a capacity retention of 80.6%. Due to the improved conductive network, 71.9% of initial capacity (1638 mAh/(g si+rco) at 0.1 A/g) was retained when the current density was increased from 0.1 A/g to 4 A/g.

Synthesis of Graphene Oxide

Graphene oxide (GO) was synthesized by Tour’s modified Hummer’s method from graphite flakes (Alfa Aesar, natural, -10 mesh, 99.9% (metals basis)). 360 ml of 98% sulfuric acid (H2SO4) and 40 ml of 97% phosphoric acid (H₃PO4) were continuously stirred in a 3-neck round bottom flask followed by the slow addition of 3g of graphite flakes and 18g of potassium permanganate (KMn04). The oxidation reaction was conducted in an oil bath at 50°C for 16 h, followed by the addition of 6 ml of hydrogen peroxide to reduce unreacted manganese species when the suspension cooled to room temperature. The resultant suspension was then washed in 10% HCI and then ethanol for three times and four times, respectively, via centrifugation (3000 rpm, rotor diameter 15 cm). Finally, a dialysis bag with 12-14 kDa molecular weight cut off (MWCO) was used to transfer GO from ethanol to deionized (Dl) water for subsequent steps.

Synthesis of Polystyrene Nanoparticles Polystyrene nanoparticles (PS NPs) were synthesized by emulsion polymerization. In a typical experiment, 72 mg of sodium bicarbonate (NaHCC>3, as buffer agent) and 9 mg of sodium dodecyl benzene sulfonate (SDBS, as emulsifier) were dissolved in 33 ml water/ethanol (10:1 in volume) in a single neck round bottom flask. 3 ml of styrene monomer was slowly injected into the mixture which was then agitated by an ultrasonic probe forming a stable white emulsion. The emulsion was then degassed by nitrogen bubbling to remove oxygen under continuous stirring for 30 mins at room temperature. After the reaction system was slowly heated to 80°C, 30 mg of potassium persulfate (KPS) (20 mg/ml in water) was slowly injected to initiate the polymerization for 7 hours under nitrogen flow protection. The resulting emulsion was broken by adding 2 g of sodium chloride (NaCI), and then washed by Dl water and ethanol four times and three times, respectively, via centrifugation (3000 rpm, rotor diameter 15 cm) and re-dispersion of the pellet containing the PS.

Preparation of Electrode Materials

500 mg of GO and 300 mg Si NPs (-50-100 nm, Strem) were dispersed in 1 L of water/ethanol (4:1 by volume). PS NPs were added with four different mass ratios (0:1, 0.5:1, 1:1 and 2:1 of PS:Si) under continuous stirring. The uniformly mixed suspension was ultrasonicated in a water bath for 30 mins, followed by spray drying using a BUCHI-290 Mini Spray Dryer with input temperature of 200°C, aspirate rate of 100%, and nitrogen flow rate of 742 L/h. The resulting dry powder was placed in an alumina combustion boat and heated under a flowing mixture of argon (95%) and hydrogen (5%) gases from room temperature to 450°C for 2 hours with a ramping rate of 1°C/min, and then heated at 800°C for 3 hours after ramping to this temperature at 5°C/min. The resulting voided crumpled reduced graphene oxide encapsulated Si NPs with varying PS/Si mass ratios are denoted as VCrGO@Si-0, VCrGO@Si-0.5, VCrGO@Si-1 and VCrGO@Si-2 herein based on the amount of PS from low to high. The sample before heat treatment is denoted herein as CGO@Si-X, where X=0, 0.5, 1 and 2.

When necessary, to attain high quality SEM images and eliminate charging of the non- conductive CGO@Si material, partial reduction under 250°C for 2 hours with a ramping rate of 1°C/min in Ar/H₂ was carried out.

Materials Characterization

Scanning electron microscope (SEM) images were taken on a field emission scanning electron microscope (Zeiss LEO1550) with an acceleration voltage of 10 kV. Thermogravimetric analysis (TGA, Q500-TA Instruments) was performed by heating the sample under air or nitrogen flow from room temperature to 650°C at a rate of 5°C/min. Transmission electron microscope (TEM) images were taken on an energy- filtered transmission electron microscope (Zeiss Libra 200MC) with an acceleration voltage of 200 kV. The size distributions of polystyrene nanospheres were obtained by counting at least 200 particles in TEM images. A CMOS detector for electron energy loss spectroscopy (EELS) integrated into the TEM was used for elemental mapping. Hydrodynamic radii were determined by dynamic light scattering (DLS, Zetasizer Nano-ZS90, Malvern). The density of PS and Si NPs was calculated based on the volume measured by a gas pycnometer (AccuPyc II, micromeritics) using helium gas. X-ray photoelectron spectroscopy (Thermal Scientific K-Alpha XPS spectrometer, 150 eV) was carried out to analyze surface elemental composition and chemical bonds. X-ray diffraction (XRD) was carried out using an XRG 3000 X-ray diffractometer (Cu Ka radiation). FEI Titan 80- 300 LB was used to obtain the high-resolution TEM (HRTEM) image of VCrGO@Si-1. The pore size distributions of various samples were measured by a dynamic vapor sorption instrument (DVS, Surface Measurement Systems) and the corresponding specific surface area was calculated based on Brunauer-Emmett-Teller (BET) theory.

Electrochemical Characterization

Electrodes were cast by mixing active material (Si + rGO), carbon black (MTI Corp., TIMCAL Graphite and Carbon Super P® Conductive Carbon Black) and sodium alginate (Sigma Aldrich) in Dl water with a mass ratio of 65:20:15 using a rotor/stator homogenizer. The resulting slurry was cast onto copper foil by a typical film casting doctor blade method and then was dried at 80°C under vacuum overnight. The mass loading of active material (Si + rGO) was adjusted to around 1 mg/cm² for all electrochemical measurements (except for FIG. 10(b) where the mass loading was increased to 2.4 mg/cm²). Coin-type half cells were assembled with lithium metal foil (Sigma Aldrich, 99.9% trace metal basis) in an Ar-filled glove box (< 1 ppm O2 and water). A Whatman glass microfiber (Grade GF/A) was used as a separator, and 1 M LiPF₆ in a 1:1 v/v mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) containing 5 vol.% fluoroethylene carbonate (FEC) purchased from Canrd China was used as electrolyte. All cells were cycled in the voltage range of 0.01 V - 1.5 V versus Li/Li+ using a LANHE multi-channel battery tester (Wuhan LAND Electronics Co.). Electrochemical impedance spectroscopy (EIS) was carried out on a SP-300 potentiostat (BioLogic) in the range of 1 MHz to 100 MHz with an AC amplitude of 10 mV. Nyquist plots were recorded after the first full cycle (after one discharge and one charge at 0.1 A/g) or after 200 cycles at 1 A/g. The electrodes were charged (delithiated) to 1.5 V, disconnected from the battery tester and connected to the potentiostat where the open-circuit voltage (OCV) was around 1.2 V for each sample. EIS was then carried out at this DC voltage vs. lithium metal.

FIG. 3 illustrates (a) size distribution of PS nanospheres by counting 200 particles in TEM images (inset), (b) TGA derivative curves of varying samples, (c) TGA of pure silicon under I h flow, and (d) TGA results of PS/Si spray dried from 3 batches of feeding solution containing PS NPs and Si NPs with a designed mass ratio of 1:1. As shown in FIG. 3(a), the average diameter of PS NPs prepared is approximately 95 ± 8 nm, which is comparable to the size of commercial Si NPs (-100 nm). FIG. 4 illustrates relative size and thermal stability of precursor and composite materials: (a) distribution of hydrodynamic diameter as estimated by DLS for GO, Si NPs and PS NPs, corresponding TEM image for PS and SEM images of Si and GO are shown as inset, (b) TGA profiles for pure PS and CGO@Si-1 and SEM image (inset) of CGO@Si-1 which was partially reduced at 250°C to render the sample conductive for easier imaging. The hydrodynamic diameter of PS NPs, Si NPs and GO sheets was also estimated by dynamic light scattering (DLS in FIG. 4(a)) which indicated average diameters of -101 nm, -89 nm and -1022 nm, respectively, in the same liquid system (ethanol/water mixture with a volume ratio of 1 :4). The similar sizes of the PS NPs and Si NPs suggest that the two particles will have a similar diffusivity, which is significantly higher than that of the GO sheets. This is important in designing the core-shell structure where it is known that larger GO sheets will mostly accumulate at the droplet interface during drying while the smaller NPs are able to redistribute while the aerosol dries and forms the core. Moreover, the convection induced during the drying process may also pin the GO at the interface where it acts to lower the surface energy. This process is illustrated in FIG. 1(b).

A scanning electron microscope (SEM) image of the VCrGO@Si-1 before removing PS template (CGO@Si-1) is shown in FIG. 4(b) inset. All composite particles display a similar crumpled structure to what has previously been reported for neat and cargo-filled GO. The lack of any obvious PS or Si on the outside of the crumpled GO indicates that most of the NPs were successfully encapsulated within the CGO framework. Thermogravimetric analysis (TGA) was carried out on both the PS NPs and CGO@Si-1 under nitrogen flow as shown in FIG. 4(b) in order to mimic the thermal reduction procedure under inert conditions (where the corresponding derivative curves are shown in FIG. 3(b)). The PS NPs slowly degrade starting around 300°C and are completely decomposed beyond ~430°C. For CGO@Si-1 samples, an earlier mass loss is observed between 150°C and 225°C which is attributed to the thermal reduction of GO with a more significant mass loss observed at higher temperatures corresponding to the PS decomposition. As a control group, TGA was carried out on pure Si NPs under similar conditions (FIG. 3(c)). There was no significant mass loss under nitrogen protection. Pure PS/Si mixtures made by spray drying (without GO) with a mass ratio of 1:1 from three separate batches were also analyzed by TGA and found to contain 53.0 ± 0.3wt% (error estimated as the standard deviation) of silicon as shown in FIG. 3(d). The slight deviation from the designed 1:1 mass ratio is attributed to experimental error and the slight oxidation of Si NPs beyond 400°C which is apparent in the TGA curves (FIG. 3(d)) as a mass increase between 400 °C and 600 °C.

FIG. 5 is a characterization of the void space created by removing PS: (a) TEM images of VCrGO@Si prepared using varying mass ratio of PS/Si, (b) HRTEM images of VCrGO@Si-1 at varying magnifications, (c) EELS elemental mapping of VCrGO@Si-1 for carbon, oxygen and silicon respectively and (d) pore size distributions of VCrGO@Si prepared using varying mass ratio of PS/Si. Transmission electron microscope (TEM) images for the high temperature annealed VCrGO@Si material are provided in FIG. 5(a) for various initial PS/Si ratios. Under TEM, rGO is relatively transparent due to its atomically-thin nature and low atomic number. Thus, regions with brighter contrast indicate empty space where the electron-beam is not blocked by the dense silicon particles. It is clear from FIG. 5(a) that there is a transition from a dense core to one that becomes more transparent to the electron beam as the proportion of void space is increased. The inner structure of VCrGO@Si-1 was better revealed using high- resolution TEM (HRTEM) imaging.

Images for a single composite ball at varying magnifications are shown in FIG. 5(b). Relatively low magnification images reveal spatially separated Si NPs. At higher magnification, it becomes clear that crystalline Si NPs (d=0.31 nm as indicated by white arrows, which belongs to Si (111)) are completely encapsulated by a layer of carbon which is attributed to multilayered crumpled rGO sheets with a thickness of around 15 nm in the region labeled by white dashed lines. The spatial distribution is better revealed by the electron energy loss spectroscopy (EELS) mapping as shown in FIG. 5(c). As expected, the carbon signal is generated from the entire structure while a faint signature of oxygen and a stronger signal for silicon reveal the inner structure of silicon within the core, suggesting that void space is successfully created. Si NPs do not appear to aggregate significantly within the VCrGO capsule after the PS template is removed. The pore size distributions of VCrGO@Si with varying PS/Si ratios were determined by dynamic vapor sorption (DVS) using cyclohexane as the organic adsorbent and plotted in FIG. 5(d). Without the addition of PS, VCrGO@Si only exhibited one sharp peak at -2 nm with a broad shoulder extending to ~5 nm, which includes the voids created by partially restacked rGO in the shell and voids between inter-particle and particle-rGO contacts. The small pore size suggests that Si NPs are compressed tightly and wrapped by rGO due to the strong capillary forces known to be generated during the rapid drying of the aerosol. After the addition of PS with a mass ratio of 0.5:1 PS to Si, a shoulder, now at ~3 nm, is significantly larger. This may be attributed to the thermal decomposition and evaporation of PS during the heat treatment. The vapor generated likely led to some expansion between the Si NPs themselves or Si NPs and inner rGO framework. Additionally, a broad hump can be observed in the range of -40-90 nm which is attributed to the void space created by the removal of the PS NPs. Wth the increased amount of sacrificial PS, more void space was introduced inside which resulted in the emergence of higher intensity peaks in this region which gradually broadened and shifted towards larger sizes. The specific surface area (SSA) was estimated based on BET theory, which was found to be 41.9 m²/g, 35.9 m²/g, 34.7 m²/g and 36.8 m²/g for VCrGO@Si-0, VCrGO@Si-0.5, VCrGO@Si-1 and VCrGO@Si-2, respectively. Since the removal of PS was conducted at a very slow heating rate (1°C/min) and long isothermal time (2 hours) at 450°C, the decomposition rate of PS is likely low and should not significantly impact the structure of the CrGO framework resulting in only minute changes in SSA. To approximate the void space introduced, the density of the as-prepared polystyrene nanospheres was determined by pycnometer using helium gas to be -1.06 g/cm³. This is around half of the density of Si nanoparticles (-2.27 g/cm³). Based on the PS/Si mass ratio designed for VCrGO@Si-0, VCrGO@Si-0.5, VCrGO@Si-1 and VCrGO@Si-2, the theoretical void space created by burning off the polystyrene template to buffer the volume expansion of silicon is estimated to be 0%, 107.1% (-100%), 214.2% (-200%) and 428.3% (-400%), respectively. FIG. 6 is an XPS survey of (a) various samples and (b) pure silicon nanoparticles. X-ray photoelectron spectroscopy (XPS) was carried out to analyze the surface elemental composition for pure CGO@Si-1 , CrGO@Si-1 and pure PS NPs as shown in FIG. 6(a). Both the carbon 1s peak (at -284 eV) and oxygen 1s peak (at -532 eV) are present at the surface of all samples. Only two low intensity peaks belonging to Si 2s (at -149 eV) and Si 2p (at -99 eV) arise from the inside of the structure despite their composition being 28.9 at% and 37.1 at% in the bulk in both CGO@Si-1 and VCrGO@Si-1. This is due to the low penetration depth (-10 nm) intrinsic to XPS which limits analysis to the surface of the crumpled structure. PS is theoretically composed of aromatic and aliphatic carbon as well as hydrogen, the 1.9 at% of oxygen presented in the measurement should be attributed to the impurities introduced during the sample preparation. CGO@Si-1 only contains 2.5 at% of silicon which is much lower than that at the surface of pure silicon (47.5 at% as shown in FIG. 6(b)) and its C/O (-2.13) is consistent with well-oxidized GO (-2.15) which can be produced by the same method. These results provide further evidence that most PS and Si NPs are successfully encapsulated and since the XPS signal is dominated by the signature of the crumpled GO shell. The Si content increased to 6.3 at% after the reduction of GO and removal of PS. To better understand the surface chemical composition, the C 1s narrow scan spectra were examined for pure PS, CGO@Si-1, and the annealed VCrGO@Si-1. FIG. 7 shows X-ray characterization of VCrGO@Si: high- resolution XPS narrow scan spectra of CGO@Si-1 (a), VCrGO@Si-1 (b) and pure PS (c) for their C 1s peaks, (d) XRD patterns of varying samples. The C 1s spectra of CGO@Si-1 (FIG. 7(a)) and the annealed VCrGO@Si-1 (FIG. 7(b)) can be deconvoluted into four peaks at around 284.8 eV, 286.9 eV, 287.9 eV, and 288.8 eV, which belong to C-C bonds, C-0 bonds, C=0 bonds, and C(0)0H bonds, respectively. After thermal reduction, the amount of all oxygen-containing functional groups decreased, particularly the C=0 and C-0 bond. As this is a surface-sensitive technique, this is indicative of graphene oxide (GO) transformation into reduced graphene oxide (rGO). As depicted in FIG. 7(c), the C 1s peak of pure PS can be deconvoluted into three peaks. The main peak at 284.8 eV includes the aliphatic and aromatic carbon bond while the small shoulder at 286.36 eV indicates C-O-C or C-OH groups coming from surface impurities. The satellite peak at 292 eV is attributed to a shake-up structure originating from TT®TT* excitations in the aromatic ring structures. X-ray diffraction (XRD) profiles for pure PS, CGO@Si-0, CGO@Si-1 and CrGO@Si-1 are plotted in FIG. 7(d), the sharp and intense peaks presented at 28.4°, 47.3°, 56.1° and 69.1° in all XRD patterns except for pure PS were assigned to (111), (220), (311) and (400) lattice planes of crystalline silicon. The large and broad peak at 2Q ~15°-23° in the XRD pattern of pure PS indicates its amorphous structure. This peak emerged in the XRD pattern of CGO@Si-1 suggesting the successful introduction of PS NPs within the spray dried crumpled structure. After being thermally annealed from CGO@Si-1 to VCrGO@Si-1, this broad peak disappeared while a broad and nearly indiscernible peak emerged at around 2Q ~20°-27° attributed to the characteristic (002) carbon peak indicating that PS NPs were removed and the rGO shell is nearly X-ray amorphous. The electrochemical performance of each of the materials containing different amounts of void space were then tested as the active material for battery electrodes using half-cells cycled against lithium metal. The rate performance was tested by progressively increasing charging rates between 0.1-4 A/g (-C/20-2C). FIG. 8 illustrates electrochemical performance of varying samples: (a) electrode capacity at various rates, (b) Nyquist plots after 1st cycle and equivalent circuit (inset), (c) charge/discharge curves for VCrGO@Si-1 at varying current densities and (d) Nyquist plots of VCrGO@Si-1 after the 1st cycle and the 200th cycle (all measurements were conducted based on a mass loading (Si+rGO) of 1 mg/cm²). As shown in FIG. 8(a), VCrGO@Si-1 displayed the best performance, achieving 1638 mAh/(g si_+rco) at 0.1 A/g and retaining 1179 mAh/(g si₊rco) at 4 A/g with a capacity retention of 71.9%. For the VCrGO@Si-0 sample (no PS template), the capacity at 1 A/g was similar but dropped considerably at 4 A/g while VCrGO@Si-0.5 can still maintain 63.2% of capacity, suggesting that the PS addition had a significant and beneficial impact on the high rate performance. However, for the sample with 400% void space (VCrGO@Si-2), the rate performance was not as good at 4 A/g, delivering only 61.7% of the capacity achieved at 0.1 A/g. This indicates that excess void space may reduce the number of effective electrical and/or ionic (for Li+ ions) contacts between the Si particles and the shell. The Nyquist plots are presented in FIG. 8(b) for all four samples. They exhibit a semi-circle in the high frequency region and a straight line in the low frequency region, which are attributed to charge transfer resistance (R_{c t}) and Warburg diffusion process (i_w), respectively. The slope of all straight lines at the low frequency region is obviously larger than 45° suggesting a deviation from ideal semi-infinite diffusion condition, which might be attributed to the short solid-state diffusion lengths in small Si NPs and additional capacitive behavior caused by the complex porous structure of the VCrGO framework. The smallest semi-circle in the curve of VCrGO@Si-1 indicates it has the lowest charge transfer resistance (58.6 W), leading to lower internal resistance when compared to VCrGO@Si-0 (87.9 W). The improvement in both rate capability and charge transfer resistance may be attributed to an improved radial distribution of GO within the microdroplet driven by the addition of PS during the spray drying as shown in FIG. 9, a schematic illustration of an improved conductive network: radial spray drying components concentration distributions of VCrGO@Si-0 (1) and

VCrGO@Si-1 (2) at stage® (a) and stage© (b). As water evaporates from the aerosol droplet in the step 1 (® to © in FIG. 1), the GO sheets may become trapped at the air-water interface while NPs attempt to maintain a uniform concentration. The initial size of the microdroplet should remain constant under the same spray drying parameters as the droplet size is controlled by the flow rate of carrier gas in the two fluid nozzles as well as the liquid viscosity and surface tension - which do not change significantly upon addition of the PS NPs. The presence of more material within the core (the additional PS NPs) could prevent some of the GO from reaching the interface, causing some material to be trapped in the core. The hypothesized radial distributions of each component for VCGO@Si-0 and VCGO@Si-0.5 before and after step 1 are shown in FIG. 9(a) and (b), suggesting VCGO@Si-0.5 theoretically has a larger GO/Si ratio in the core region compared to the sample without PS. As silicon is a poorer electronic and lithium ion conductor, after heat treatment, contacting the rGO instead of another Si NP would improve the overall conductivity. It can be surmised that the void space, introduced via PS removal, is able to effectively buffer the volume expansion of silicon and maintain a good conductive network during charge/discharge. On the other hand, excess void space made the charge and/or ion transfer route too long, resulting in a larger Ret (75.6 W) of VCrGO@Si-2. The voltage profiles in terms of specific capacity of VCrGO@Si-1 are plotted in FIG. 8(c), showing a typical shape for lithiation (discharge) and de-lithiation (charge) process of silicon at 0.2 A/g with the plateaus at ~ 0.3 V - 0.01 V and -0.2 V - 0.6 V, respectively. With the increasing current density from 0.2 A/g to 4 A/g, the polarization of Si led to an apparent increase in the de-lithiation potential and reduction of the lithiation potential.

FIG. 10 illustrates cyclic stability of varying samples: (a-c) HRTEM images of VCrGO@Si-1 at varying magnifications, (d-g) EELS elemental mapping of VCrGO@Si-1 for carbon, oxygen and silicon respectively (all measurements were conducted based on a mass loading (Si+rGO) of 1 mg/cm² except for FIG. 10(b)). The cycle-life of all samples was investigated by applying a 0.1 A/g current for the 1st cycle and a 1 A/g current for the following 200 cycles, which were plotted in FIG. 10(a). VCrGO@Si-1 displayed the best cyclic stability, which only dropped from 1468 mAh/(g si₊rco) to 1183 mAh/(g si₊rco) with a capacity retention of 80.6% compared to that of VCrGO@Si -0 (61.3%) and VCrGO@Si-2 (79.5%). VCrGO@Si-1 also displayed an improved columbic efficiency (70.6%) of the 1^st cycle which is much higher than that of pure silicon (57.2%), suggesting that the crumpled rGO shell with mesopores can partially prevent Si NPs from directly being exposed to electrolyte. In addition, the cycle life of VCrGO@Si-0.5 is also plotted for comparison, which only displayed a minor improvement in capacity retention (64.2%) after 200 cycles. Based on the above, it can be concluded that VCrGO@Si with a mass ratio of PS/Si at 1:1 possesses the most appropriate void space to buffer the volume expansion of Si during cycling. FIG. 11 shows the TGA result of VCrGO@Si-1. According to the TGA result, the content of rGO in VCrGO@Si-1 is around 43.6 wt%. FIG. 12 provides the analysis of the capacity contribution of VCrGO@Si-1. As shown in FIG. 12, since pure VCrGO can provide a capacity of -294 mAh/(g _rco) at 1 A/g, the capacity per gram of VCrGO@Si-1 contributed from Si core is around 1340 mAh at 1 A/g (the real specific capacity of Si core is -2376 mAh/(g si)). In theory, the pure silicon anode can display a theoretical capacity of 3590 mAh/g with a volume expansion of -300% at room temperature. Considering only -66% of theoretical capacity is displayed, the average real volume expansion can be assumed as -198%. VCrGO@Si with a mass ratio of PS/Si at 1:1 can theoretically create a void space to buffer -200% volume expansion. This designed theoretical value of VCrGO@Si-1 is just above the estimated volume expansion of the Si core at 1 A/g, which explains the reason why VCrGO@Si-1 displayed the best electrochemical performance. FIG. 14 shows SEM images of VCrGO@Si-1 after 200 cycles at varying magnifications. As shown in FIG. 14(a), the density of VCrGO@Si should decrease with the increasing amount of sacrificial PS template. The density is estimated to 1.5 g/cm³, 1.28 g/cm³, 1.13 g/cm³ and 0.91 g/cm³ for VCrGO@Si-0, VCrGO@Si-0.5, VCrGO@Si-1 and VCrGO@Si-2, respectively. To compare these four samples, their capacities were also converted into a volumetric capacity as shown in FIG.

14(b). Even with a lower electrode density, VCrGO@Si-1 still displayed the best volumetric capacity of 1337 mAh/cm³ at 1 A/g after 200 cycles, which is around 75.33% of the theoretical volumetric capacity for this composite (1774 mAh/cm³) estimated by excluding the volume of extra void space (i.e., using the density of VCrGO@Si-0).

The EIS results were recorded for VCrGO@Si-1 after the 1^st cycle and the 200th cycle and plotted in FIG. 8(d). The Nyquist plots maintain a similar shape and only a small expansion is observed for the semi-circle in the high frequency region, while obvious growth in both ohmic resistance (Ro) and Ret can be observed for VCrGO@Si-0 (FIG. 13, which shows Nyquist plots of VCrGO@Si-0 after the 1st cycle and the 200th cycle), suggesting the void space reserved crumpled rGO shell can help silicon based anodes maintain a good electrochemical structure after long-term cycling. For VCrGO@Si-0, after 200 cycles, the emergence of an additional semi-circle at higher frequencies may indicate resistance through a thicker SEI that has developed as a result of cycling due to the absence of enough void space in this sample.

In addition, SEM images of VCrGO@Si-1 before and after cycling were taken as shown in FIG. 14, no significant cracks were observed and still the crumpled core-shell structure was maintained, suggesting most of the volume expansion of Si is buffered in the rGO shell. To better confirm the improvement of Si anodes using the VCrGO protective layer, the mass loading of active materials (Si + rGO) was increased from 1 mg/cm² to 2.4 mg/cm². With such a high loading, VCrGO@Si-1 displays an initially high areal capacity of 2.26 mAh/cm² which maintains >80% of capacity after 145 cycles. To compare with other works in the technical literature based on methods of creating void space to buffer volume expansion of Si/C anodes, the main electrochemical performance has been summarized in FIG. 15 and Table 1 below.

Table 1. Summary of recent works about Si/C with artificially or naturally reserved void space. Particles manufactured according to the exemplary embodiment exhibit competitive cycle life and areal mass loading with a relatively simple preparation method compared to the above. For electrodes in Table 1 meeting the following conditions: (1) capacity retention ³ 90% after 100 cycles and (2) mass loading ³ 1 mg/cm², their capacity at varying current densities are recorded in FIG. 16, suggesting that particles manufactured according to the exemplary embodiment also displayed a favourable rate capability compared to the prior art.

In summary, polystyrene nanospheres were synthesized by emulsion polymerization with a diameter of 85 ± 8 nm and used as sacrificial material which can be thermally removed at 450°C in inert gas atmosphere. Then, PS NPs were spray dried together with Si NPs and GO sheets, followed by the reduction of GO and the removal of PS in tube furnace. The mass ratio of PS/Si is set as 0:1 , 0.5:1 , 1 :1 and 2:1 for comparison, aiming to buffer the volume expansion of 0%, 100%, 200% and 400%, respectively. According to various electrochemical tests,

VCrGO with a PS/Si ratio of 1:1 was determined as the optimal composition exhibiting desirable electrochemical performance in terms of minimizing internal resistance, increasing tolerance to high rate cycling, and maintaining long-term cyclic stability when compared to bare Si or encapsulated Si (with no PS). VCrGO@Si-1 retained around 80.6% of the initial capacity after 200 cycles while VCrGO@Si-0 only retained 61.3%. At a higher active material loading of 2.4 mg/cm², VCrGO@Si-1 displayed a high areal capacity of 2.26 mAh/cm² which only dropped by <20% after 145 cycles. Based on the calculation for the capacity contributed by pure silicon (94.3%) at 1 A/g, the reserved void space of VCrGO@Si-1 may effectively buffer the volume change of Si NPs in the core, without introducing excess volume, improving overall performance.

Exemplary Embodiment Using Hydrogel as Electrochemically Inactive Particles

In a second exemplary embodiment of the present invention, crumpled rGO encapsulated Si nanoparticles are prepared with reserved void space upon the swelling/shrinkage of carboxymethyl cellulose - poly (ethylene oxide) (CMC-PEO) hydrogel, which is denoted as rGO/gel/Si. A schematic illustration of the basic synthesis process of rGO/gel/Si is depicted in FIG. 17, where the orange region in (b) represents non-cross-linked hydrogel and the purple region in (c-e) represents cross-linked hydrogel, involving mixing and spray-drying CMC/PEO polymer chains, citric acid (CA), GO sheets, and Si nanoparticles. Due to the size differences amongst the varying components during the aerosol evaporation, a quasi-core-shell structure forms (FIG. 17(b)) where the core is composed of Si NPs, CMC/PEO chains, citric acid, and a small quantity of GO sheets while the shell mainly contains partially restacked crumpled GO sheets. The resulting composite core-shell structures are heated at 80°C to initiate the cross- linking reaction between CMC and PEO where citric acid is used as the cross-linker. The CMC/PEO hydrogel containing composite (denoted as GO/gel/Si) can then be immersed in water to make the inside hydrogel fully swell resulting in increased overall volume enclosed by the GO shell, followed by chemical reduction to remove oxygen-containing functional groups from GO and lock the shell into place via the stronger van der Waals forces and tt-p stacking. Finally, the rGO/gel/Si is slowly dried to dehydrate and shrink the hydrogel which then remains as an elastic and Li-ion conducting binder, leading to the separation of Si NPs from the crumpled rGO framework. The reserved void space can provide such a material with a desirable cycle life, which only drops by -18.3% of capacity from 1055 mAh/(g rco+cei+si) at 1 A/g after 320 cycles.

Synthesis of Graphene Oxide

GO was synthesized using Tour’s modified Hummer^'s method, which exhibits a C/O of -2.1. In a typical preparation process, 3 g of graphite flakes (Alfa Aesar, 10 mesh, 99.9%) and 18 g of potassium permanganate (98% KMn0₄, Alfa Aesar) were slowly added into the acid mixture, which is composed of 360 ml of 98% sulfuric acid (H2SO4) and 40 ml of 97% phosphoric acid (H₃PO4). The oxidation reaction was conducted at 50°C for 16 h under continuous stirring, and the resulting thick purple slurry was poured into 400 g of ice and cooled down to room temperature. Around 6 ml of hydrogen peroxide (H2O2) was then slowly added until the color of the suspension completely turned to a golden or yellow colour. After being washed with 10% hydrochloric acid (HCI) and ethanol for 3 times and 4 times, respectively, via centrifugation (3000 rpm, rotor diameter 15 cm), the GO suspension was transferred from ethanol to deionized water (Dl water) using a dialysis bag with 12-14 kDa molecular weight cut off (MWCO).

Preparation of Pure CMC/PEO Hydrogel

Sodium CMC and PEO powder (Sigma Aldrich, 419303, MW -250,000) and PEO powder (Sigma Aldrich, 182001, MW -300,000) with a mass ratio of 3:1 were dissolved in Dl water with a concentration of 4% (w/v). Then, citric acid with concentrations of 10% and 20% (w/w) compared to CMC/PEO was added into the CMC/PEO homogenous solution and then stirred for 12 h. The resulting thick solution was poured into an aluminum dish (diameter: 6cm) and dried at 40°C for 6 h. The dried CMC/PEO hydrogel was punched into discs and cross-linked at 80°C for 6 h.

Preparation of Void Space Reserved rGO/Gel/Si

500 mg of GO and 200 mg of amorphous Si NPs (~ 50 - 100 nm, Strem) were mixed in 1000 ml of Dl water during water bath ultrasonication. CMC/PEO (3:1 in weight) was dissolved in GO/Si dispersion with a concentration of 20% (w/w) compared to GO. Then, citric acid with a concentration of 10% wt% compared to CMC/PEO was added and stirred for 12 h. The uniformly mixed suspension was then spray dried using a BUCHI-290 Mini Spray Dryer with input temperature of 100°C, aspirator set to 100% and a nitrogen flow rate of 600 L/h. The gel encapsulated within the dry powder was cross-linked by heating at 80°C for 6 h and the powder was then immersed in 50 ml_ of Dl water in a round-bottom flask at a concentration of 3 mg of powder/ml for 12 h. Chemical reduction to convert the GO to rGO was then conducted by placing the flask in an oil bath at 80°C for 12 h under continuous stirring, and hydrazine solution (35 wt%) was added as a reducing agent (1 pi for 3 mg of reactant). The final product was dried in a vacuum oven at 40°C for 12 h and denoted as rGO/gel/Si. Spray dried gel and silicon, denoted as gel/Si, was prepared in the same way without the addition of GO. Non spray dried rGO/gel/Si was prepared using air drying at 40°C to substitute the spray drying process. rGO/gel/Si (10%) was prepared in the same way but adding 10% (w/w) of CMC/PEO rather than 20%.

Material Characterization

Scanning electron microscope (SEM) images were taken on a field emission scanning electron microscope (Zeiss LEO1550) with an acceleration voltage of 10 kV. The size distributions of GO/gel/Si and GO/Si crumpled balls were obtained by counting at least 200 particles in SEM images by ImageJ. Thermogravimetric analysis (Q500, TA Instruments) was performed by heating the sample under air flow from room temperature to 650°C at a rate of 5°C/min. Transmission electron microscope (TEM) images were taken on an energy-filtered transmission electron microscope (Zeiss Libra 200MC) with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (Thermal Scientific KAIpha XPS spectrometer, 150 eV) was carried out to analyze the surface elemental composition and chemical bonding. An FEI Titan 80-300 LB was used to obtain the high-resolution TEM (HRTEM) and high-angle annular dark- field (HAADF) images of rGO/gel/Si. A CMOS detector for electron energy loss spectroscopy (EELS) integrated into the TEM was used for elemental mapping. The depth profiling of rGO/gel/Si was detected by Auger Microprobe (MicroLab 350) with an argon ion gun.

Electrochemical Characterization

The working electrode was prepared by mixing active material, carbon black (Super P, MTI), and sodium alginate (Sigma Aldrich) in Dl water with a mass ratio of 65:20:15 using a rotor/stator homogenizer. The resulting slurry was cast onto copper foil by a typical film casting doctor blade method, followed by drying at 80°C under vacuum overnight. All rate capability and cyclic stability results were obtained by assembling the fabricated working electrode (~ 1 mg/cm² of active material (rGO + gel + Si) for all studies, except for FIG. 26(b) (where ~ 2.5 mg/cm² of active material was applied) with a lithium metal foil (Sigma Aldrich, 99.9% trace metal basis) in a coin-type half-cell. A Whatman glass microfiber (Grade GF/A) was used as a separator, and 1 M LiPF₆ in a 1:1 v/v mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) containing 5 vol.% FEC purchased from Canrd China was used as electrolyte. All cells were assembled in an Ar-filled glove box (< 1 ppm O2 and water) and cycled between 0.01 V and 1.5 V versus Li/Li+ using a multi-channel battery tester (Wuhan LAND Electronics Co.). For the full-cell measurement, ~6.7 mg/cm² of LiFeP0₄ (LFP, Gelon Chemical) was cast onto aluminum foil, where polyvinylidene difluoride (PVDF, Kynar® HSV 900 Arkema) and carbon black were used as binder and conductive additive with a weight ratio of 80:10:10 in N-methyl-2-pyrrolidone (NMP). The cells were then assembled using the same aforementioned electrolyte composition and glovebox. The test voltage window for full cells is 2 V - 3.9 V by using the same battery tester. Electrochemical impedance spectroscopy (EIS) was carried out on a SP-300 potentiostat (BioLogic) in the range of 1 MHz to 100 MHz with an AC amplitude of 10 mV. Nyquist plots were recorded after the first full cycle (after one discharge and one charge at 0.1 A/g). The electrodes were charged (delithiated) to 1.5 V, disconnected from the battery tester and connected to the potentiostat where the OCV was around 1.2 V for each sample. EIS was then carried out at this DC voltage vs. lithium metal.

FIG. 19 illustrates characterization of hydrogel cross-linking and swelling/shrinkage: (a) Fourier- transform infrared spectroscopy (FTIR) patterns of gel/Si (without GO) before and after the heat treatment at 80°C and cross-linking, CMC/PEO (3:1) hydrogel containing 10 wt% of citric acid before (b) and after (c) being immersed in Dl water for 24h, (d) swelling ratios in diameter and weight of varying hydrogels, size distributions of GO/Si (e) and GO/gel/Si (f) before and after being immersed in water for 12h, (g) FTIR patterns of rGO/Si (after cross-linking and high temperature reduction) with and without hydrogel, and TEM images of rGO/Si (h) and rGO/gel/Si (i). To reveal the bonding environment and confirm the success of cross-linking, FTIR was conducted on gel/Si without GO before and after cross-linking as shown in FIG.

19(a) (SEM images of spray dried pure gel/Si are shown in FIG. 18). The peak at around 1704 cm^·1 corresponds to the ester linkage of cross-linking, whose intensity significantly increases after heating the gel/Si at 80°C for 12 h suggesting CMC polymers are successfully cross- linked by citric acid. Although the residence time for a single microdroplet in the spray dryer is estimated in the range of -0.1-10ms, the outlet temperature of the spray dryer must be set above 40°C which causes the gel/Si to be partially crosslinked and results in an observable peak in the curve of the non-crosslinked sample. Furthermore, an increased peak intensity is observed at -3456 cm^·1 (-OH stretching) due to increased intramolecular and intermolecular hydrogen bonding which has been shown to occur after the crosslinking reaction. This can also be attributed to moisture trapped inside the cross-linked hydrogel network. FIG. 19(b) and (c) show photographs of cross-linked CMC/PEO with 10 wt% citric acid before and after immersion in water for 24 h. Immersion resulted in immediate CMC/PEO swelling with an equilibrium (meaning no further change in size is observed) reached after 6 h. This hydrogel is able to maintain its shape without dissolution during the full 24 h immersion. The diameter of the hydrogel pellet increases from 14.6 mm to 23.0 mm, displaying a swelling ratio of 58% in diameter and 355% in weight (as summarized in FIG. 19(d)). When the concentration of citric acid increases to 20 wt%, the swelling ratios in diameter and weight drop to 14% and 63%, respectively. Since each citric acid molecule can bridge at most two polymer chains either between the same or different polymers, the increased amount of citric acid leads to the formation of stronger bonding and a tighter crosslinked network which limits the swelling in water. However, it was confirmed that CMC chains mainly participate in the crosslinking reaction while PEO chains are simply trapped in the hydrogel network enhancing the swellability. For both of these two well-cross-linked hydrogel pellets, the swelling behavior was found to be reversible as the pellet shrunk to its original dimensions after being dried and expanded again after it was reimmersion in water. This highlights the flexibility and elastomeric properties of CMC within the gel which is widely used as an effective aqueous-based electrode binder for silicon materials. In contrast, non-cross-linked CMC/PEO hydrogel swelled quickly in water and finally completely dissolved in water after 24 h (see FIG. 20, images of non-cross- linked CMC/PEO (10 wt% citric acid) before (a) and after (c), cross-linked CMC/PEO (20 wt% citric acid) before (b) and after (d) immersed in water for 24h). Significant swelling of the hydrogel was only observed in water rather than the EC/DMC (1:1 v/v) battery electrolyte which allowed for reserving void space via particle volume adjustment in water which was retained when the material was exposed to the electrolyte. Considering its large swelling ratio and desirable structural stability, the cross-linked CMC/PEO hydrogel containing 10 wt% citric acid was selected for use in the following electrode preparation to better produce void space with a small amount.

Si NPs in suspension exhibit a much smaller hydrodynamic radius compared to that of GO sheets (~ 41 nm vs. ~ 660 nm on average, which was estimated by dynamic light scattering), suggesting Si NPs have much larger diffusivity according to the Stokes-Einstein relation. The difference between Si NPs and GO sheets in both dimension and diffusivity is important in designing the quasi-core-shell structure as shown in FIG. 17(b). During spray drying, most GO sheets accumulate at the air/water interface due to the shrinkage of water droplets while the Si NPs and likely some smaller GO sheets are able to redistribute into the core. Since the dissolved CMC, PEO, and citric acid are even smaller and thus exhibit a higher diffusivity, these hydrogel components are able to redistribute within the core to maintain a uniform concentration. SEM images of spray dried GO/Si and GO/gel/Si are shown in FIG. 21 (SEM images of GO/Si before (a) and after (b), GO/gel/Si before (c) and after (d) immersion in water for 12h), and all materials display a similar crumpled, spherical shape with GO wrinkles distributed about the surface. Only a few, and in most cases no, Si NPs are observed in these images suggesting the majority of particles, along with the gel, are successfully encapsulated within the crumpled GO. The size distributions for GO/Si and GO/gel/Si are depicted in FIG. 19(e) and (f), respectively. After being immersed in water for 12h, the average diameter of GO/gel/Si increases from 1006 (± 33%) nm to 1353 (± 31%) nm yielding a swelling ratio, based on diameter, of 34.5%, while that of GO/Si only increases from 856 (± 32%) nm to 919 (± 29%) nm with a swelling ratio of 7.4% in diameter. The increased swelling ratio for the GO/gel/Si material is attributed to the water uptake resulting in the formation of the hydrogel. However, when the content of gel is increased from 20 wt% (with respect to the mass of GO) to 40 wt%, the crumpled rGO shells effectively burst open leading to the Si NPs being released from the shell after hydrazine reduction and then settled down during the subsequent centrifugation and washing steps as shown in FIG. 22 ((a) image of rGO/gel/Si (40%), where Si came out and settled down to the bottom during centrifugation washing, (b) and (c) SEM images of resulting material from (a) where crumpled rGO framework is broken). This suggests there is an optimum gel content (and resulting void space produced) which is dictated by the rGO shell strength when a reasonable silicon content (> 50 wt% compared to rGO) remained. Hence, the rGO/gel/Si or GO/gel/Si only indicates 20 wt% of gel unless otherwise specified. FTIR was also used to confirm cross-linking between CMC/PEO occurred within the crumpled GO. As shown in FIG. 19(g), the newly emerged 1704 cm ¹ peak in the curve of rGO/gel/Si indicates that CMC/PEO polymers are successfully cross-linked by citric acid and introduced into the rGO framework. The enhanced peak at 1595 cm^·1 is caused by more carboxylate anion (COO) coming from CMC and PEO. Similarly, the larger peak at 1060 cm ¹ which belongs to C-0 stretching also suggests the successful introduction of cross-linked CMC/PEO hydrogel.

A TEM image of an rGO/Si particle without the addition of hydrogel is shown in FIG. 19(h), the uniformly dark contrast across most of the particles suggesting a dense packing of the silicon which prevents transmission of the electron beam, whereas the edge reveals the appearance of transparent sheet-like structures with wrinkles corresponding to the rGO wrapping around the Si aggregates. The TEM transparency of rGO stems from its low atomic number and thin structure consisting of single- to few-layer defective graphene. After the addition of the hydrogel (FIG. 19(i)) and being fully dried under vacuum, the particles become more transparent to the electron beam and more features of rGO are revealed around regions of darker contrast which likely belong to dense Si NPs and shrunken hydrogel. Moreover, these dark regions are concentrated at one side, illustrating that the shrinkage of hydrogel leads to the partial separation of the Si/gel core from the rGO shell. As a control, the GO/gel/Si was reduced by hydrazine vapor without being immersed in water. In this case, the hydrogel does not swell and no significant void space should be introduced. A corresponding TEM image is shown in FIG. 30 where no obvious electron transparent regions can be observed suggesting that a dense core-shell structure results.

FIG. 23 illustrates high-resolution characterization of the rGO/gel/Si inner structure: (a) HRTEM at varying magnifications, (b) HAADF image of rGO/gel/Si and (c) overlay of C and Si elemental maps. The inner structure was more clearly investigated by using HRTEM as shown in FIG. 23(a) at varying magnifications. At low magnification, the inner void space can be divided into two types: 1) interparticle void space and 2) the void space between the rGO edge and Si/rGO aggregates. The former should be attributed to the breakage of the hydrogel while the latter is caused by the separation/shrinkage of hydrogel from the the rGO shell during drying. This is consistent with what was observed for the pure hydrogel, the full shrinkage during vacuum drying usually accompanied by the emergence of cracks. At higher magnification, no obvious lattice fringes can be found in the region of Si NPs and the quasi transparent part of the rGO exhibits an amorphous feature. At the wrinkled region of the rGO, some lattice fringes can be observed, suggesting some restacking of rGO sheets. These wrinkles and ridges formed by capillary consolidation during spray drying are known to stabilize the crumpled structure and lead to the compression and aggregation resistance of the crumpled balls. EELS was conducted to better reveal the elemental distribution in the crumpled rGO/gel/Si structure. By comparing the HAADF image of rGO/gel/Si (FIG. 23(b)) with elemental maps for C, O, Na and Si shown in FIG. 24 (EELS elemental mapping for C (a), Na (b), O (c) and Si (d) respectively), we see carbon uniformly distributed about the structure suggesting that a 3D crumpled rGO framework forms and wraps all of the other components within. A small amount of Na from the NaCMC is also detected (FIG. 24(b)) and distributes uniformly about the crumpled framework suggesting the hydrogel is able to completely fill the core space due to its strong hydrophilic interaction between both Si NPs and GO sheets. Considering NaCMC is a common binder used in commercial Si/C anodes, the hydrogel cannot only create the void space by swelling/shrinkage but also provides an elastic network to enhance robustness of the whole crumpled structure. The distribution of O and Si are localized within the shell as shown in FIG. 24(c) and (d). In particular, the spatially separated distribution of silicon is clearly demonstrated in FIG. 23(c) by overlapping the C and Si EELS maps.

XPS was carried out to analyze the surface chemical composition and bonding information for GO/Si, GO/gel/Si, and rGO/gel/Si as shown in FIG. 25 (characterization of varying samples: (a- c) high-resolution XPS survey spectra, high-resolution XPS narrow scan spectra of GO/Si (b), GO/gel/Si (c) and rGO/gel/Si (d), (e) AES depth profiling of rGO/gel/Si, (f) TGA results for pure CMC/PEO hydrogel and rGO/gel/Si conducted under air flow). All samples display two sharp and intense peaks at -284 eV and -532 eV which belong to the carbon 1s and oxygen 1s peaks, respectively. Two peaks belonging to Si 2s (at -149 eV) and Si 2p (at -99 eV) are very small in rGO/gel/Si and become negligible in GO/Si and GO/gel/Si, suggesting that the removal of functional groups on the surface of GO might expose more signal from silicon. However, since the composition of silicon in the bulk of the rGO/gel/Si is around 21.5 at% according to EDS results (which, unlike XPS, can probe several microns into the sample), it can be confirmed that most Si NPs are wrapped inside. The C/O ratio of the crosslinked GO/gel/Si (-1.92) is slightly smaller than the GO/Si (-2.01), indicating that some of the polymer constituents of the hydrogel might adsorb to the surface of GO during spray drying because the C/O ratios of all components are less than 2, and the cross-linking upon esterification caused further loss of oxygen. Moreover, the new nitrogen 1s peak emerged in rGO/gel/Si at -400 eV, which is caused by the well-known nitrogen doping of the rGO caused by hydrazine reduction. High resolution C 1s spectra for GO/Si and GO/gel/Si materials (see FIG. 25(b)-(d)) can be deconvoluted into four peaks at around 284.8 eV, 286.9 eV, 287.9 eV, and 288.8 eV, which belong to C-C bonds, C-0 bonds, C=0 bonds, and C(0)0H bonds, respectively. The slight increase in peak area of C-0 and C=0 bonds indicates that residual hydrogel components are adsorbed to the surface of the crumpled rGO shell. Considering the strong hydrophilic interaction between GO and hydrogel due to the large number of oxygen-containing functional groups (e.g., -OH and -COOH), some hydrogel components might be adsorbed or entrapped at the surface or between the layers of crumpled rGO shell during spray drying since they can be easily dissolved in water at the molecular level. After immersion in water and hydrazine reduction, the surface of rGO still contains a large amount of oxygen-containing bonds and the C/O ratio only increased from -1.92 to -5.6 according to XPS results. This moderate reduction in efficiency compared to the chemical reduction of GO sheets using the same reducing agent can be attributed to the complex crumpled structure, leading to the decay of reduction efficiency of hydrazine for GO sheets shielded by the surface of GO sheets in the crumpled rGO framework. This can be also attributed to the volume expansion of the hydrogel which partially unfolds the crumpled rGO and potentially exposes more polymers to the surface. To better characterize the radial elemental distribution within the crumpled rGO/gel/Si composite, a single core-shell structure was depth profiled using argon-sputtering and AES. FIG. 25(e) plots the elemental intensity ratio of C, O, and Si as a function of sputter time, and electrons were focused on a single crumpled composite ball as shown in FIG. 26 (SEM images of the single rGO/gel/Si ball which is conducted for depth profiling measurement). The content of both oxygen and silicon increases while the carbon content keeps dropping, approaching a constant value after being sputtered for 260s, confirming expectations that most of the GO sheets are concentrated at the surface while other components (Si, gel and a small amount of GO) remain inside after spray drying. Even after swelling, the chemically reduced rGO/gel/Si still maintained a similar radial elemental distribution. TGA analysis was conducted on both the pure gel and rGO/gel/Si as shown in FIG. 25(f) to determine the mass fraction of silicon in this composite. Two significant mass drops are observed in the curve for the pure gel at -220- 300°C and ~400-450°C which are attributed to the thermal decomposition of the hydrogel. Similar behavior is detected for rGO/gel/Si, but the loss at ~400-450°C is significantly larger because rGO was also burned off at this temperature range. Since the mass ratio of Si to hydrogel is 2:1, the mass fraction of silicon is calculated based on mass retentions in FIG. 25(f) and determined as ~ 38.1 wt% Si while the gel occupies around 19.1 wt%. In the following electrochemical measurements, all specific capacities are reported based on the total mass of rGO, gel and Si.

FIG. 27 illustrates electrochemical performance of varying samples: (a) rate capability and (b) Nyquist plots of rGO/Si and rGO/gel/Si, (c) voltage profiles of rGO/gel/Si in terms of capacity at varying current densities, (d) discussion of the capacity contribution from varying components to rGO/gel/Si (all electrochemical measurements are based on the mass loading (rGO+gel+Si) of 1 mg/cm²). The capacity vs. charge/discharge rate for electrodes fabricated from rGO/gel/Si and rGO/Si are plotted in FIG. 27(a). The rGO/gel/Si samples display an improved capacity retention (64.1%) at high rates compared to that of rGO/Si (48.6%), which only drops from 1333 mAh/(g rco+gei+si) at 0.1 A/g to 854 mAh/(g _rGo+_gei+si) at 4 A/g. The Nyquist plots are presented in FIG. 27(b) for these two samples. All curves exhibit a semi-circle in the high- frequency region and a straight line in the low-frequency region, which are attributed to charge transfer resistance (R_ct) and Warburg-like diffusion processes (i_w). The slopes of all straight lines in the low-frequency region are larger than 45° suggesting deviation from the ideal semi infinite diffusion condition, which might be attributed to the additional capacitive behavior caused by the complex porous structure of the crumpled rGO framework. The rGO/gel/Si shows a much smaller semi-circle compared to that of rGO/Si, indicating a smaller R_ct which could explain the improved performance at high rates. The addition of the hydrogel can improve the electronic and ionic charge transport in the following ways: 1) PEO is well-known as a solid electrolyte used in lithium-ion batteries which may facilitate ionic conduction to/from the silicon within the core; 2) Incorporating the gel components in the spray drying dispersion may impact the distribution of GO/rGO within the core resulting in an improved 3D network of electronically conducting rGO. The voltage profiles in terms of specific capacity of rGO/gel/Si at varying current densities are plotted in FIG. 27(c), showing the typical shape for lithiation (discharge) and de-lithiation (charge) processes for silicon at 0.2 A/g with the plateaus at ~ 0.3 V - 0.01 V and - 0.2 V - 0.6 V, respectively. With increasing current density from 0.2 A/g to 4 A/g, the polarization of Si leads to an apparent increase in the de-lithiation potential and reduction of lithiation potential. The capacity contribution of all components from the active materials of rGO/gel/Si is illustrated in FIG. 27(d). The weight percentage of gel, rGO, and Si can be estimated based on the TGA results (FIG. 25(f)), which are approximately 19, 43, and 38 wt%, respectively. Assuming the capacity contribution from pure gel is zero and hydrazine reduced pure rGO displayed a capacity of -200 mAh/(g _rco) at 1 A/g, the capacity of 1 gram rGO/gel/Si contributed by rGO is only 38.1 mAh and by Si is 1016.9 mAh. Hence, Si nanoparticles within the core displayed a true capacity of 2669.03 mAh/(g si) at 1 A/g, which is around -74.34% of its theoretical capacity at room temperature.

The cyclic stability of various samples was measured at 1 A/g for 320 cycles after a forming cycle at 0.1 A/g. FIG. 28 illustrates electrochemical performance of various samples: (a) cyclic stability of rGO/Si and rGO/gel/Si prepared at varying conditions, (b) cyclic stability of rGO/gel/Si at varying mass loadings of active materials (all electrochemical measurements based on the mass loading (rGO+gel+Si) of 1 mg/cm² except for FIG. 28(b)). As shown in FIG. 28(a), rGO/gel/Si displays the best cycle life, which only drops from 1055 mAh/(g _rGo+_gei+si) to 862 mAh/(g rco+gei+si) with a capacity retention of 81.7%. As control groups, the capacity rGO/gel/Si without spray drying and rGO/Si rapidly decay to 530 mAh/(g _rGo+_gei+si) (-52.6% retention) and 564 mAh/(g rco+gei+si) (-49.3% retention) after 320 cycles, indicating that the best long-term performance is achieved when the void space was engineered between the crumpled rGO shell and the gel/Si core. Moreover, the gel can also work as the binder to enhance the robustness of the crumpled quasi-core-shell structure. Even after 320 cycles, the impedance of the rGO/gel/Si electrode was not significantly increased, as shown in FIG. 31.

To improve the overall capacity, the amount of gel added during sample preparation was decreased from 20 wt% to 10 wt% (compared to the weight of GO). While the overall capacity improved, the resulting material only retained - 62.5% of its capacity after the 320 cycles. Increasing the volume of hydrogel beyond 40 wt% introduces stresses on the rGO framework leading to unwrapping or bursting of the GO shell and thus could not be tested. Hence, rGO/gel/Si with 20 wt% of CMC/PEO addition forms an optimized core exhibiting the best electrochemical performance. Even after the mass loading of active materials (rGO+gel+Si) is increased from 1 mg/cm² to 2.5 mg/cm², rGO/gel/Si still retains around 79% of the initial capacity (~ 2.1 mAh/cm²) after 200 cycles.

To further evaluate the rGO/gel/Si in practical batteries, full-cells where LiFeP0₄ (LFP) was used as the cathode material were fabricated. The exemplary full-cell displayed excellent cycle life as shown in FIG. 29(a), which only dropped by 6.7% from 109 mAh/(g

after 100 cycles at 1 A/g in terms of the weight of rGO/gel/Si. This is the first time that the void space between a carbon shell and silicon core has been achieved by taking advantage of the swelling/shrinkage of a hydrogel in hydrated vs. dehydrated states. As shown in FIG. 29(b), when the areal capacity achieved is compared to recent literature, which also engineer silicon anodes with void space, the exemplary core-shell structure with a gel core displayed competitive cyclic stability.

In summary, the foregoing presents a novel method to improve the structural stability and electrochemical performance of Si/C based anodes. PEO and CMC polymer chains were successfully cross-linked and wrapped inside a crumpled rGO framework with Si NPs. This process takes advantage of the innate feature of the CMC/PEO hydrogel which swells in water and shrinks after drying thereby expanding the volume of GO framework during water immersion. The void space between rGO shell and gel/Si core is then introduced by drying in air, meanwhile, the GO was chemically reduced. The resulting void space was visually demonstrated by HRTEM and EELS elemental mapping. With the optimized structure, rGO/gel/Si displayed a significantly improved cyclic stability, which only dropped by 18.3% from 1055 mAh/(g _rco_+gei+si) after 320 cycles at 1 A/g. Although the weight content of silicon is only 38.1 wt%, around 96.4% of capacity is contributed by silicon with a real capacity of 2669.03 mAh/(g _Si).

The present invention comprises a composite electrode structure where a protective graphene shell is wrapped around clusters of active material and a method to introduce void space within the core of such structures using a third component which is mixed with the active material and used to control void space within the core. This allows the active material to expand/contract during charge/discharge without causing a significant strain on the graphene shell. This significantly improves the cycle-life of thick electrodes such as silicon anodes. It has been found that void space can be engineered in the core of the crumpled graphene shell by incorporating various polymers into the graphene/silicon dispersion to be spray dried. In one case, we found that polystyrene (PS) nanoparticles, of similar size to the silicon, are uniformly distributed in the core and can be removed by a subsequent heat treatment step. The space that was originally taken up by the PS becomes void space for the silicon to expand/contract upon charge and discharge. By varying the amount of PS to silicon, an optimal ratio of 1:1 was determined which maximized the cycle-life and surprisingly also improved the rate capabilities of the anodes fabricated. In a second case, we incorporated a hydrogel (polyethylene oxide/carboxymethyl cellulose) into the core of the crumpled graphene shell using the same procedure. Void space was created by placing the composite structure in water and allowing the hydrogel to swell. Upon drying, the hydrogel shrunk and created void space for the silicon to expand/contract upon charge/discharge. Polystyrene nanoparticles can be substituted by other template materials which can be fully removed during heat treatment or any carbon source which can be thermally carbonized with volume shrinkage. However, these should be similar in size to the silicon in order to remain well-distributed in the core. Some options might include carbonates or bicarbonates which can be easily gasified/removed by heat treatment, other polymers like polypropylene, polyethylene, polymethylmethacrylate, etc., which typically decompose without residue or polymers which partially decompose and may leave behind a conductive carbonaceous residue such as chitosan, polyacrylonitrile, gum Arabic, etc.

Other components such as carbon nanotubes or additional polymer binders can be added to the core or shell to improve electrical or ionic conductivity or mechanical properties of the composite.

The present invention may be useful for other battery materials similar to silicon which expand significantly upon charge/discharge such as germanium, tin, etc.

The foregoing is considered as illustrative only of the principles of the present invention. The scope of the claims should not be limited by the exemplary embodiments set forth in the foregoing, but should be given the broadest interpretation consistent with the specification as a whole.

Claims

1. A composite particle for use as a battery electrode precursor material, comprising a lithium-permeable graphene-based external layer defining an inner volume, the inner volume containing a plurality of electrochemically active particles and electrochemically inactive particles, the electrochemically inactive particles capable of volume reduction to form void space within the inner volume.

2. The composite particle of claim 1 wherein the electrochemically active particles are selected from the group consisting of silicon, silicon oxide, tin, germanium, antimony, TiO, ZnO, SnO, COO, FeO, MnO, NiO, MoO, MoC, CuO, Cu₂0, Ce0₂, RuO, carbon, a bimetallic material, a multi-metallic material, an oxide material, a sulfide material, and combinations thereof.

3. The composite particle of claim 1 wherein the electrochemically inactive particles comprise at least partially thermally decomposable material selected from the group consisting of polystyrene, PEG, NaHC0₃, NH₄HCO₃, PMMA, sucrose, wheat particles, starch, saw dust, chitosan, polyacrylonitrile, and gum Arabic.

4. The composite particle of claim 1 wherein the electrochemically inactive particles comprise a responsive hydrogel susceptible to volume decrease such that the responsive hydrogel shrinks to form the void space.

5. The composite particle of claim 1 wherein the battery electrode is a silicon anode.

6. The composite particle of claim 5 wherein the electrochemically active particles comprise silicon nanoparticles which expand contract during charge and discharge.

7. The composite particle of claim 6 wherein the void space allows the silicon nanoparticles to expand into the void space to reduce stress on the graphene-based external layer.

8. The composite particle of claim 3 wherein the electrochemically inactive particles comprise polystyrene nanoparticles.

9. The composite particle of claim 4 wherein the responsive hydrogel is a carboxymethyl cellulose - poly (ethylene oxide) hydrogel.

10. The composite particle of claim 1 wherein the graphene-based external layer comprises graphene oxide nanoparticles.

11. The composite particle of claim 1 wherein the electrochemically active particles are of similar size to the electrochemically inactive particles.

12. The composite particle of claim 10 wherein the electrochemically active particles are of similar size to the electrochemically inactive particles, and the graphene oxide nanoparticles are larger than the electrochemically active particles and the electrochemically inactive particles.

13. A method for manufacturing a battery electrode precursor material, comprising the steps of: a. forming a dispersion of graphene and electrochemically active particles; b. combining electrochemically inactive particles capable of volume reduction with the dispersion; c. forming the dispersion into particles with the graphene as an external layer; and d. treating the particles to cause the electrochemically inactive particles to reduce in volume to form void space within the particles.

14. The method of claim 13 wherein the step of forming the dispersion of the graphene and the electrochemically active particles comprises mixing the graphene and the electrochemically active particles in a water/ethanol mixture.

15. The method of claim 13 wherein the electrochemically active particles are selected from the group consisting of silicon, silicon oxide, tin, germanium, antimony, TiO, ZnO, SnO, COO, FeO, MnO, NiO, MoO, MoC, CuO, CU2O, Ce0₂, RuO, carbon, a bimetallic material, a multi- metallic material, an oxide material, a sulfide material, and combinations thereof.

16. The method of claim 13 wherein the electrochemically inactive particles comprise at least partially thermally decomposable material selected from the group consisting of polystyrene, PEG, NaHC0₃, NH₄HCO₃, PMMA, sucrose, wheat particles, starch, saw dust, chitosan, polyacrylonitrile, and gum Arabic.

17. The method of claim 13 wherein the electrochemically inactive particles comprise a responsive hydrogel susceptible to volume decrease such that the responsive hydrogel shrinks to form the void space.

18. The method of claim 17 wherein the responsive hydrogel is a carboxymethyl cellulose - poly (ethylene oxide) hydrogel formed by cross-linking of the carboxymethyl cellulose with the poly (ethylene oxide) by citric acid as a cross-linker in the presence of heat.

19. The method of claim 13 wherein the step of forming the dispersion into particles comprises spray-drying the dispersion after combining the electrochemically inactive particles.

20. The method of claim 16 wherein the step of treating the particles comprises applying heat to the particles to at least partially decompose the electrochemically inactive particles to form the void space.

21. The method of claim 20 wherein the graphene is graphene oxide and the step of applying heat reduces the graphene oxide to reduced graphene oxide.

22. The method of claim 17 further comprising the step after the step of forming the dispersion into the particles of hydrating the particles with water to swell the responsive hydrogel in the particles.

23. The method of claim 22 wherein the step of treating the particles comprises evaporating at least some of the water to shrink the responsive hydrogel to form the void space.

24. The method of claim 13 further comprising selecting a ratio of electrochemically active particles to electrochemically inactive particles to selectively tune the amount of the void space that will be formed by the step of treating the particles.