BATTERIES WITH ANODES OF CARBON-COATED MACRO-POROUS SILICON

Abstract

Silicon materials suitable for use as an anode material and associated method of production are disclosed herein. In one embodiment, a silicon material includes crystalline silicon in a matrix and macro-scale pores distributed in the matrix of the crystalline silicon. The macro-scale pores can have a size greater than 100 nanometers, and surfaces of crystalline silicon in the macro-scale pores are coated with carbon.

Claims

1. A method of forming a silicon material for use in anodes of batteries, the method comprising: reacting a metal or a mixture of multiple metals with silicon (Si) in a solid state reaction to form a metal silicide; performing a CO.sub.2-thermic oxidation process on the formed metal silicide with carbon dioxide to form a composite of one or more metal oxides, silicon, and carbon; and contacting the formed composite of one or more metal oxides, silicon, and carbon with an acid to remove the one or more metal oxides from the composite to produce particles of the silicon material individually having: a crystalline silicon formed in the CO.sub.2-thermic oxidation process; and pores in the crystalline silicon, the pores corresponding to the removed metal oxide by the acid and having surfaces coated with the carbon from the performed CO.sub.2-thermic oxidation process, wherein reacting the metal or the mixture of multiple metals with silicon includes reacting one of magnesium (Mg), calcium (Ca), or barium (Ba) with silicon (Si); and performing the CO.sub.2-thermic oxidation process includes performing the CO.sub.2-termic oxidation process with one of magnesium silicide (Mg.sub.2Si), calcium (CaSi.sub.2), and barium silicide (BaSi.sub.2) and wherein in case Mg is present, Mg is at least 10 wt % more in excess of silicon.

2. The method of claim 1 wherein reacting the metal or the mixture of multiple metals with silicon includes reacting the metal or the mixture of multiple metals with silicon (Si) in a solid state reaction in an inert reaction environment.

3. The method of claim 1 wherein: reacting the metal or the mixture of multiple metals with silicon includes performing a solid state reaction between the metal or the mixture of multiple metals and silicon with the metal or the mixture of multiple metals being in excess of stoichiometric proportion, thereby forming the metal silicide along with excess metal or mixture of metal; and performing the CO.sub.2-thermic oxidation process includes reacting the excess metal or mixture of metal to form additional one or more metal oxides and additional carbon.

4. The method of claim 1 wherein performing the CO.sub.2-thermic oxidation process includes i) performing the CO.sub.2-thermic oxidation process at a reaction temperature of 600° C. to 800° C. for a reaction period of 50 minutes to 600 minutes or ii) flowing a gas of the carbon dioxide across the formed metal silicide at a reaction temperature of 600° C. to 800° C. for a reaction period of 50 minutes to 600 minutes, or iii) flowing a gas of the carbon dioxide across the formed metal silicide at a reaction temperature of 600° C. to 800° C. for a reaction period of 50 minutes to 600 minutes to form the composite of the one or more metal oxides, silicon, and carbon, wherein the one or more metal oxides being distributed in the silicon in the formed composite, and wherein a surface of the silicon is coated generally uniformly with the formed carbon.

5. The method of claim 1 wherein: reacting the metal or the mixture of multiple metals with silicon includes reacting at least two from the group consisting of magnesium (Mg), calcium (Ca), and barium (Ba) with silicon (Si); and performing the CO.sub.2-thermic oxidation process includes performing the CO.sub.2-thermic oxidation process with two from the group consisting of magnesium silicide (Mg.sub.2Si), calcium silicide (CaSi.sub.2), and barium silicide (BaSi.sub.2).

6. The method of claim 1 wherein: reacting the metal or the mixture of multiple metals with silicon includes i) reacting magnesium (Mg) with silicon (Si) to form magnesium silicide (Mg.sub.2Si) as follows:
2Mg+Si.fwdarw.Mg.sub.2Si; performing the CO.sub.2-thermic oxidation process includes performing the CO.sub.2-thermic oxidation process with the magnesium silicide (Mg.sub.2Si) to form a composite of magnesium oxide (MgO), silicon, and carbon, as follows:
Mg.sub.2Si+CO.sub.2.fwdarw.Si+C+2MgO; and contacting the formed composite with the acid includes contacting the formed composite with the acid to remove the magnesium oxide (MgO) from the formed composite or ii) reacting calcium (Ca) with silicon (Si) to form calcium silicide (CaSi.sub.2) as follows:
Ca+2Si.fwdarw.CaSi.sub.2; performing the CO.sub.2-thermic oxidation process includes performing the CO.sub.2-thermic oxidation process with the calcium silicide (CaSi.sub.2) to form a composite of calcium oxide (CaO), silicon, and carbon, as follows:
2CaSi.sub.2+CO.sub.2.fwdarw.4Si+C+2CaO; and contacting the formed composite with the acid includes contacting the formed composite with the acid to remove the calcium oxide (CaO) from the formed composite to produce particles with a layered two-dimensional silicon morphology.

7. A battery, comprising: a first electrode; an electrolyte in electrical communication with the first electrode, the electrolyte containing a plurality of metal ions; and a second electrode spaced apart from the first electrode and in electrical communication with the first electrode via the electrolyte, wherein the second electrode contains particles of a silicon material individually having: the crystalline silicon in a matrix; and pores distributed in the matrix of the crystalline silicon, the pores having a size greater than 100 nanometers, wherein surfaces of the crystalline silicon in the pores are coated with carbon, wherein the silicon material is obtainable by a method according to any one of claim 1 to 6.

8. The battery of claim 7 wherein the surfaces of the crystalline silicon in the pores are coated with carbon generally uniformly.

9. The battery of claim 7 wherein: the pores are randomly distributed in the matrix of the crystalline silicon; and the surfaces of the crystalline silicon in the pores are coated with carbon generally uniformly, in particular, the pores are randomly distributed in the matrix of the crystalline silicon; and the surfaces of the crystalline silicon in the pores are coated with carbon generally uniformly, the carbon being in an amorphous form, a semi-crystalline or crystalline form, or a mixture thereof.

10. The battery of claim 7 wherein: the first electrode is constructed from one of lithium cobalt oxide (LiCoO.sub.2), lithium iron phosphate (LiFePO.sub.4), lithium manganese oxide (LiMn.sub.2O.sub.4), lithium nickel oxide (LiNiO.sub.2), lithium iron phosphate fluoride (Li.sub.2FePO.sub.4F), LiNiCoAlO.sub.2 having 80% Nickel, 15% Cobalt, and 5% Aluminum, LiCoO.sub.2 having 100% Cobalt, LiMn.sub.2O.sub.4 having 100% Manganese, LiNiMnCoO.sub.2 having 33.3% Nickel, 33.3% Manganese, and 33.3% Cobalt, LiFePO.sub.4 having 100% Iron, or lithium cobalt nickel manganese oxide (LiCo.sub.1/3Ni.sub.1/3Mn.sub.1/3O.sub.2); and the electrolyte contains LiPF.sub.6, LiAsF.sub.6, LiClO.sub.4, LiBF.sub.4, or lithium triflate.

11. The battery according to any one of claims 7 to 10 wherein the silicon material is obtained according to a method of claim 6.

12. A silicon material according to claim 1, comprising: the crystalline silicon in a matrix; and pores distributed in the matrix of the crystalline silicon, the pores having a size greater than 100 nanometers, wherein surfaces of the crystalline silicon in the pores are coated with carbon.

13. The silicon material of claim 12 wherein the surfaces of the crystalline silicon in the pores are coated with carbon generally uniformly.

14. The silicon material of claim 12 wherein: the pores are randomly distributed in the matrix of the crystalline silicon; and the surfaces of the crystalline silicon in the pores are coated with carbon generally uniformly, in particular, wherein: the pores are randomly distributed in the matrix of the crystalline silicon; and the surfaces of the crystalline silicon in the pores are coated with carbon generally uniformly, the carbon being in an amorphous form, a semi-crystalline or crystalline form, or a mixture thereof.

15. The silicon material of claim 12 wherein: the pores are randomly distributed in the matrix of the crystalline silicon; the surfaces of the crystalline silicon in the pores are coated with carbon generally uniformly, the carbon being in an amorphous form, a semi-crystalline or crystalline form, or a mixture thereof; and the matrix of the crystalline silicon has a layered two-dimensional silicon morphology.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIGS. 1A and 1B are schematic diagrams showing an example battery incorporating an anode having a carbon-coated macro-porous silicon material in accordance with embodiments of the disclosed technology.

[0012] FIG. 2 is a schematic diagram illustrating an example CO.sub.2-thermic oxidation process suitable for forming the carbon-coated macro-porous silicon material in FIGS. 1A and 1B.

[0013] FIGS. 3A-3D are example scanning electron microscope (SEM) images of corresponding products from each stage of the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0014] FIG. 4A shows analysis results of X-ray diffraction (XRD) of example products of a CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0015] FIG. 4B shows analysis results of N.sub.2 adsorption-desorption analysis of example products of the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0016] FIGS. 5A and 5B are example transmission electron microscopy (TEM) images of spherical macro pores and carbon layers on a silicon surface and carbon layer (dotted area) on silicon matrix of example products formed using the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0017] FIGS. 5C-5E are example energy-dispersive X-ray spectroscopy (EDS) elemental mapping results showing generally homogeneous distribution of carbon throughout a silicon particle formed using the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0018] FIG. 5F shows an example Raman spectroscopy results before and after acid leaching showing that MgO is effectively removed and existence of carbon in a silicon particle formed using the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0019] FIGS. 6A-6C are schematic diagrams showing example reaction kinetics between gaseous CO.sub.2 and Mg.sub.2Si during initial bulk diffusion of gaseous CO.sub.2, surface diffusion causing early formation of MgO/Si/C composite, and subsequent reaction between Mg.sub.2Si and diffused CO.sub.2 in accordance with embodiments of the disclosed technology.

[0020] FIG. 7A shows an example first cycle voltage profile of a battery incorporating a carbon-coated macro-porous silicon material formed using the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology. In particular, FIG. 7A shows lithiation and delithiation capacities of about 2732 mA.Math.h/g and 2273 mA.Math.h/g, respectively, which resulting in >83.2% coulombic efficiency. FIG. 7A also includes an inset showing dQ/dV plot for first cycling showing sharp peaks at ˜0.05V, indicating phase transition of crystalline Si to amorphous Si.

[0021] FIG. 7B shows an example rate capability result exhibiting ˜1000 mA.Math.h/g specific capacity at 1.0 C-rate and recovering back to ˜2000 mA.Math.h/g at 0.2 C-rate of a battery incorporating a carbon-coated macro-porous silicon material formed using the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0022] FIG. 7C shows example representative voltage profiles for C-rate of 0.2, 0.5, 0.8, and 1.0 of a battery incorporating a carbon-coated macro-porous silicon material formed using the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0023] FIG. 7D shows a comparison of 0.2 C-rate cycling performance among example bulk silicon material, silicon via air-oxidation of Mg.sub.2Si, and carbon-coated macro-porous silicon from the CO.sub.2-thermic oxidation process in accordance with embodiments of the disclosed technology.

[0024] FIG. 7E shows example lithiation-delithiation profiles of carbon-coated macro-porous silicon material at different cycles in accordance with embodiments of the disclosed technology.

[0025] FIG. 8A shows an example SEM image of a layered silicon/carbon composite in accordance with embodiments of the disclosed technology.

[0026] FIG. 8B shows an example result of an ex-situ XRD analysis showing that major phase is crystalline Si with approximately <10 wt % of FeSi.sub.2 believed due to the existence of Fe in a starting material in accordance with embodiments of the disclosed technology.

[0027] FIG. 8C shows an example cycling performance of a layered two-dimensional silicon/carbon composite at 0.5 C-rate in accordance with embodiments of the disclosed technology.

[0028] FIG. 9A shows an example in-situ high temperature XRD (HT-XRD) analysis of Mg.sub.2Si during the CO.sub.2-thermic oxidation process. In particular, FIG. 9A shows phase evolution of MgO and Si as well as the decrease of Mg.sub.2Si peak intensity starting at around 500° C.

[0029] FIG. 9B shows an example peak Intensity change during in-situ HT-XRD showing clear phase evolution of Si and MgO in accordance with embodiments of the disclosed technology.

[0030] FIGS. 9C-9E show example magnified in-situ HT-XRD peaks at around 2θ=28˜29°, 42˜44°, and 39˜40° where evident change in Si, MgO, and Mg.sub.2Si peak intensities, respectively, can be seen with respect to temperature changes.

[0031] FIG. 10 shows example Gibbs free energies at various temperatures for Mg.sub.2Si formation and subsequent CO.sub.2-thermic oxidation process of Mg.sub.2Si compared with Air (or O.sub.2) oxidation in accordance with embodiments of the disclosed technology.

[0032] FIGS. 11A and 11B show example ex-situ XRD analysis to investigate the influence of temperature (600˜800° C.) and holding time (50˜600 minutes) on complete conversion of Mg.sub.2Si, respectively.

[0033] FIGS. 12A-12J show example SEM images of MgO+Si+C composite at 700° C. for various holding times of 50 min, 100 min, 200 min, 400 min, and 600 min. These figures show that overall particle size does not change much but the morphology is significantly altered by the holding time from 50 min to 400 min.

[0034] FIG. 13 shows example XRD patterns for CaO+Si+C composite after CO.sub.2-thermic oxidation process with CaSi.sub.2 (bottom) and Si+C after CaO removal (middle and top) in accordance with embodiments of the disclosed technology.

DETAILED DESCRIPTION

[0035] Various embodiments of battery systems, devices, and associated methods of making are described herein. Even though the technology is described below using a lithium-ion battery as an example, in other embodiments, the technology may be applicable in other types of batteries (e.g., containing sodium, potassium, calcium, magnesium, cadmium, or copper ions), a chemical sensor, an organic thin film transistor, an electromechanical actuator, a gas separation membrane, a fuel cell, and/or other suitable electronic components. In the following description, specific details of components are included to provide a thorough understanding of certain embodiments of the disclosed technology. A person skilled in the relevant art will also understand that the disclosed technology may have additional embodiments or may be practiced without several of the details of the embodiments described below with reference to FIGS. 1A-13.

[0036] Certain lithium-ion batteries use carbon-based materials, such as graphite, as anodes. However, such carbon-based materials tend to limit charging capacities of lithium-ion batteries. For example, lithiation of a carbon-based anode can produce a charging capacity of about 372 mA.Math.h/g, while lithiation of silicon (Si) can produce a charging capacity of about 4,200 mA.Math.h/g. However, lithiation in silicon can involve about 300% volume change. Thus, after a few charge-discharge cycles, a bulk silicon-based anode would be pulverized. Silicon anodes can also form unstable solid-electrolyte interphase layers with electrolytes in lithium batteries. As such, the poor mechanical stability and chemical passivation properties of silicon can render silicon electrodes unsuitable for practical battery systems.

[0037] Several embodiments of the disclosed technology are directed to a battery system having an anode containing a carbon-coated macro-porous silicon material. The carbon-coated macro-porous silicon material can include particles individually containing crystalline silicon having macro-scale pores of greater than about 100 nm. Both internal and external surfaces of the crystalline silicon can be coated, at least partially, with carbon of an amorphous form, a semi-crystalline or crystalline form, or a mixture thereof. Experiments have shown that batteries having anodes constructed from examples of such a carbon-coated macro-porous silicon material exhibit high electrical capacity and cyclability. In addition, examples of the silicon material can be produced using a CO.sub.2-thermic oxidation process to form crystalline silicon and the coated carbon in a single reaction, as discussed in more detail below.

[0038] FIG. 1A is a schematic diagram of an example lithium-ion battery 100 during discharging, and FIG. 1B is a schematic diagram of the lithium-ion battery 100 during charging in accordance with embodiments of the technology. As shown in FIGS. 1A and 1B, the lithium-ion battery 100 can include a container 102 holding an anode 104, a cathode 106, an electrolyte 108, and an optional membrane 110 in the electrolyte 108. Even though only certain components are illustrated in FIGS. 1A and 1B, in other embodiments, the lithium-ion battery 100 can also include insulators, gaskets, vent holes, and/or other suitable components (not shown). In further embodiments, the membrane 110 may be omitted.

[0039] The cathode 106 can be constructed from a layered oxide (e.g., lithium cobalt oxide (LiCoO.sub.2)), a polyanion (e.g., lithium iron phosphate (LiFePO.sub.4)), or a spinel (e.g., lithium manganese oxide (LiMn.sub.2O.sub.4)). Other suitable materials for the cathode 106 can include lithium nickel oxide (LiNiO.sub.2), lithium iron phosphate fluoride (Li.sub.2FePO.sub.4F), lithium cobalt nickel manganese oxide (LiCo.sub.1/3Ni.sub.1/3Mn.sub.1/3O.sub.2), Li(Li.sub.aNi.sub.xMn.sub.yCo.sub.z)O.sub.2, and/or other suitable cathode materials. Additional examples of metal compositions suitable for the cathode 106 can also include: [0040] LiNiCoAlO.sub.2 having 80% Nickel, 15% Cobalt, and 5% Aluminum [0041] LiCoO.sub.2 having 100% Cobalt [0042] LiMn.sub.2O.sub.4 having 100% Manganese [0043] LiNiMnCoO.sub.2 having 33.3% Nickel, 33.3% Manganese, and 33.3% Cobalt [0044] LiFePO.sub.4 having 100% Iron

[0045] In certain embodiments, the electrolyte 108 can include a non-aqueous solution of organic carbonates such as ethylene carbonate or diethyl carbonate containing complexes of lithium ions. The electrolyte 108 can contain non-coordinating anion salts such as LiPF.sub.6, LiAsF.sub.6, LiClO.sub.4, LiBF.sub.4, and lithium triflate. In other embodiments, the electrolyte 108 may also include an aqueous and/or a partially aqueous solution of lithium salts and/or other suitable compositions.

[0046] The anode 104 can be constructed, at least partially from a carbon-coated macro-porous silicon material. The carbon-coated macro-porous silicon material can include particles individually containing crystalline silicon having macro-scale pores of greater than about 100 nm distributed therein randomly or in other suitable manners. Both internal and external surfaces of the crystalline silicon can be coated, at least partially, with carbon of an amorphous form, a semi-crystalline or crystalline form, or a mixture thereof. Such a silicon material can be formed using a CO.sub.2-thermic oxidation process described herein. The number and/or distribution of the macro-scale pores as well as morphology of the crystalline silicon can be controlled by adjusting one or more of a selection of metal silicide or a level of excess of metal for forming the metal silicide, as discussed in more detail below with reference to FIGS. 2-13.

[0047] As shown in FIG. 1A, during discharging, lithium ions 112 are extracted from the anode 104 and migrate toward the cathode 106 via the electrolyte 108. The lithium ions 112 pass through the optional membrane 110 and are inserted into the cathode 106. As a result, a current 114 flows from the cathode 106 past a load 116 to the anode 104. As shown in FIG. 1B, during charging, a charger 118 provides a charging current 120 to the cathode 106. The charging current 120 causes lithium ions 112 to be extracted from the cathode 106 and move toward the anode 104. The lithium ions 112 pass through the optional membrane 110 and are inserted into the anode 104 via, e.g., alloying and/or otherwise combining with the material of the anode 104.

[0048] Several embodiments of the lithium-ion battery 100 can have a higher charging capacity than conventional batteries using a carbon-based material for anode. For example, as discussed above, a conventional lithium-ion battery with a carbon-based anode can have a charging capacity of about 372 mA.Math.h/g while several embodiments of the lithium-ion battery 100 can have a charging capacity of about 2,000 mA.Math.h/g. As a result, the charging capacity of the lithium-ion battery 100 may be improved when compared to conventional lithium-ion batteries.

[0049] Several embodiments of the anode 104 of the lithium-ion battery 100 can have improved charging/discharging cyclability. Without being bound by theory, it is believed that several embodiments of the anode 104 can accommodate the large volume changes associated with lithium insertion because the macro pores in the crystalline silicon can contract and expand. For example, the macro pores may contract during lithium insertion and expand during lithium extraction without causing pulverization and/or other structural damage to the anode 104 as a whole. It is also believed that the carbon coating on the silicon material can also protect silicon from forming unstable solid-electrolyte interphase layers with electrolytes in a lithium battery. As such, chemical passivation properties of the silicon material can be improved to render the anode 104 stable for practical battery systems.

[0050] FIG. 2 is a schematic diagram illustrating an example CO.sub.2-thermic oxidation process suitable for forming the carbon-coated macro-porous silicon material in FIGS. 1A and 1B. In FIG. 2, magnesium silicide (Mg.sub.2Si) was used to illustrate aspects of the disclosed technology. In other implementations, calcium (Ca), barium (Ba), or other suitable metals may also be used in addition to or in lieu of magnesium in the CO.sub.2-thermic oxidation process.

[0051] An initial stage of the CO.sub.2-thermic oxidation process can include a solid state reaction 200 between a metal and silicon. For example, as illustrated in FIG. 2, elemental magnesium (Mg) and silicon (Si) powders can be mixed to form a silicon/magnesium mixture 210. In certain embodiments, the magnesium and silicon can be in stoichiometric proportion of Mg.sub.2Si. In other embodiments, the magnesium can be in excess of stoichiometric proportion of Mg.sub.2Si to ensure full conversion of Si to Mg.sub.2Si. The solid state reaction 200 can then include heating the silicon/magnesium mixture at a reaction temperature (e.g., about 500° C.) for a period of time (e.g., about 10 hours) in an inert (e.g., argon) environment to obtain an intermetallic phase of Mg.sub.2Si 212, as follows:

2Mg+Si.fwdarw.Mg.sub.2Si

In other examples, the solid state reaction 200 also can include reacting other suitable materials to form intermetallic compounds, such as magnesium stannide (Mg.sub.2Sn) and Mg.sub.2Cu.

[0052] Subsequent to the solid state reaction 200, the CO.sub.2-thermic oxidation process can also include a thermal reaction 202 at which the formed intermetallic compound is oxidized by carbon dioxide. For example, as shown in FIG. 2, the magnesium silicide can be oxidized by carbon dioxide as follows:

Mg.sub.2Si+CO.sub.2.fwdarw.2MgO+Si+C

As shown above, products of the thermal reaction 202 can include a matrix of crystalline silicon 214 coated with carbon 216 and magnesium oxide (MgO) 218 dispersed in the crystalline silicon 214 randomly or in other suitable manners. The carbon 216 can be in an amorphous form, a semi-crystalline or crystalline form, or a mixture thereof. In embodiments in which the magnesium is excess of stoichiometric proportion of Mg.sub.2Si, the excess magnesium can react with the carbon dioxide as follows to form additional magnesium oxide 218 and carbon:

2Mg+CO.sub.2.fwdarw.2MgO+C

[0053] Upon completion of the thermal reaction 202, the CO.sub.2-thermic oxidation process can further include an acid leaching reaction 204 in which the metal oxide (e.g., MgO) is removed from the crystalline silicon 214 using hydrofluoric acid (HF), hydrochloric acid (HCl), or other suitable types of acid. Thus, as shown in FIG. 2, after the acid leaching reaction 206, the magnesium oxide is removed to form macro-scale pore 220 in the crystalline silicon 214. Both the interior and exterior surfaces of the crystalline silicon 214 can be covered with the carbon 216. As such, particles of crystalline silicon 214 can be efficiently formed with macro-scale pores and surface carbon 216 using the CO.sub.2-thermic oxidation process. Changes in morphology of various products during stages of the CO.sub.2-thermic oxidation process are discussed in more detail below with reference to FIGS. 3A-3D.

[0054] FIGS. 3A-3D are example scanning electron microscope (SEM) images of example products from each stage of the CO.sub.2-thermic oxidation process of FIG. 2 to illustrate change in morphology in accordance with embodiments of the disclosed technology. As shown in FIG. 3A, starting silicon powder exhibits smooth facets and average particle size of about 15 μm, which then drastically transforms into lumpy chunk (shown in FIG. 3B) after the thermal reaction 202 with magnesium powder. The transition from Si to Mg.sub.2Si may lead to an increase in particle size (e.g., about 20 μm). Based on the solid state reaction 200, approximately 160% of volume increase may occur during a phase transition from Si diamond cubic crystal structure with a lattice parameter of a=0.543 nm to Mg.sub.2Si antifluorite structure with a lattice parameter of a=0.634 nm.

[0055] It is believed that magnesium is a main diffusive species during the solid-state reaction 200 with silicon. In general, the substance with a lower melting point has a relatively weaker atomic bonding and higher molar volume, which allows faster diffusion than the one with a higher melting point. In the illustrated example, the melting points of Mg and Si are 650° C. and 1414° C., respectively. Thus, Mg may easily diffuse into the innermost Si matrix, and hence enables the complete transformation to the intermetallic phase of Mg.sub.2Si. Subsequently, during the thermal reaction 202, thermal oxidation of Mg.sub.2Si occurs and lead to a composite of magnesium oxide 218, crystalline silicon 214, and carbon 216.

[0056] As shown in FIG. 3C, the morphology of the composite after the thermal reaction 202 was an irregular-shaped lump with numerous speckles on the surface. During the acid leaching reaction 204, acid leaching was performed to furnish the porous structure by removing magnesium oxide 218, shown in FIG. 3D. The removed oxides, such as MgO and possibly SiOx, acted as sacrificial template for creating the macro-porous structure in the crystalline silicon 214. The macro-scale pores are believed to be desirable for Si-based anode materials and can contribute to a higher tap density. Tap density is typically measured by mechanically tapping a container until little further volume change is observed. Tap density is believed to be closely related to size and porosity of a tested material. In certain experiments, the carbon-coated macro-porous silicon material exhibited almost 3 times higher tap density than commercially available nanosized silicon powder.

[0057] FIG. 4A shows analysis results of X-ray diffraction (XRD) of example products of a CO.sub.2-thermic reaction in accordance with embodiments of the disclosed technology. The XRD analysis showed various stages of transition from silicon, to magnesium silicide, to a composite of magnesium oxide, silicon, and carbon, and finally to a silicon/carbon composite. N.sub.2 adsorption-desorption analysis performed on the example silicon material showed that the BET surface areas of samples before and after acid leaching were 15.1, and 67.6 m.sup.2/g indicating the role of magnesium oxide 218 as a sacrificial template for creating the macro-porous structure and relatively low surface area compared to typical Nano-engineered Si-based anode materials. Furthermore, the SEM and TEM high magnification images in FIGS. 5A and 5B show that the Si/C composite after acid leaching exhibits porous structures with size greater than 100 nm and a generally uniform layer of carbon on the porous crystalline silicon.

[0058] To illustrate the formation of carbon 216 via the CO.sub.2-thermic oxidation process with intermetallic compounds, EDS elemental mapping (shown in FIGS. 5C-5E) and Raman spectroscopy (shown in FIG. 5F) were performed. In the EDS result, generally uniform distribution of carbon throughout the particle were confirmed. Two peaks at around 1350/cm (D band) and 1580/cm (G band) in Raman analysis show the existence of carbon in the sample. The intensities in D and G bands are almost 1:1, which implies that the carbon synthesized through the CO.sub.2-thermic oxidation process was a mixture of amorphous (or disordered) and graphitic carbon.

[0059] The reaction mechanism between gaseous CO.sub.2 and Mg.sub.2Si may include three diffusion stages based on the behavior of gaseous CO.sub.2 throughout the conversion, as shown in FIGS. 6A-6C. Initially, as shown in FIG. 6A, CO.sub.2 molecules move towards the Mg.sub.2Si surface through the bulk gas layer. Then, as shown in FIG. 6B, surface diffusion of CO.sub.2 molecules along the exterior of Mg.sub.2Si particles occurs, and the first oxidation reaction of Mg.sub.2Si occurs, which leads to the formation of the MgO+Si+C composite at the surface. After the formation of the first surficial MgO+Si+C composite layer, the consecutive supply of CO.sub.2 gas molecules into the interior of the Mg.sub.2Si can be achieved through inter-/intra-particular diffusion of CO.sub.2 molecules through the newly formed MgO+Si+C composite layer to continuously promote the oxidation reaction with internal Mg.sub.2Si until generally a full conversion is achieved.

[0060] The electrochemical properties (e.g., cycling performance and rate capability) of example carbon-coated macro-porous silicon material produced during experiments were evaluated. As shown in FIG. 7A, initial lithiation and delithiation capacities of about 2732 mA.Math.h/g and 2273 mA.Math.h/g, respectively were observed. Such capacities correspond to about 83.2% initial coulombic efficiency. Furthermore, the carbon-coated macro-porous silicon material also showed a good rate capability (shown in FIG. 7B) of approximately 1000 mA.Math.h/g at a 1.0 C rate, which recovered back to 2000 mA.Math.h/g at 0.2 C rate. Representative lithiation/delithiation profiles at each C rate were shown in FIG. 7C, indicating phase transition of amorphous silicon during cycling. In FIGS. 7D and 7E, cycling performances at 0.2 C rate of the Si/C composite were compared with bulk Si and air-oxidized Si samples. Results showed good cycling stability of the example silicon material over 400 cycles exhibiting ˜87% capacity retention and typical lithiation/delithiation phase transition during cycling.

[0061] Test with CaSi.sub.2 via CO.sub.2-thermic oxidation process were also conducted, as shown in FIGS. 8A-8C and 13. These results showed that CaSi.sub.2 is also suitable for having a strong thermodynamic tendency to be oxidized with CO.sub.2. The final morphology of the reaction product from CaSi.sub.2 is different than the from Mg.sub.2Si possibly due to the unique crystal structure of CaSi.sub.2 resulting in buckled layered two-dimensional silicon morphology. The Si/C composite with layered morphology via CO.sub.2-thermic oxidation process with CaSi.sub.2 exhibits stable cycling performance with about 1000 mA.Math.h/g at 90.sup.th cycle (shown in FIG. 8C).

[0062] FIG. 9A shows an example in-situ high temperature XRD (HT-XRD) analysis of Mg.sub.2Si during the CO.sub.2-thermic oxidation process. As shown in FIG. 9A, phase evolution of MgO and Si as well as decrease of Mg.sub.2Si peak intensity can be clearly seen starting at around 500° C. FIG. 9B shows an example peak intensity change during in-situ HT-XRD showing clear phase evolution of Si and MgO. FIGS. 9C-9E show example magnified in-situ HT-XRD peaks at around 2θ=28˜29°, 42˜44°, and 39˜40° where evident change in Si, MgO, and Mg.sub.2Si peak intensities, respectively, can be seen with respect to temperature changes. As shown in FIGS. 9A-9E, phase evolution of MgO and Si as well as the decrease of Mg.sub.2Si peak intensity starting at around 500° C.

[0063] FIG. 10 shows example Gibbs free energies at various temperatures for Mg.sub.2Si formation and subsequent CO.sub.2-thermic oxidation process of Mg.sub.2Si compared with Air (or O.sub.2) oxidation in accordance with embodiments of the disclosed technology. As seen in FIG. 10, a thermodynamic driving force for this oxidation reaction is sufficient (ΔG=−519 kJ/mol at 700° C.), a surficial conversion of magnesium to Mg.sub.2Si may proceed readily. In a CO.sub.2 environment, the reaction is much milder exothermic than the one in air.

[0064] FIGS. 11A and 11B show example ex-situ XRD analysis to investigate the influence of temperature (600˜800° C.) and holding time (50˜600 minutes) on complete conversion of Mg.sub.2Si, respectively. As shown in FIGS. 11A and 11B, temperatures from about 650° C. to about 800° C. and holding times of about 100 minutes to about 600 minutes may be sufficient to convert Mg.sub.2Si into magnesium oxide, silicon, and carbon by reacting with carbon dioxide.

[0065] FIGS. 12A-12J show example SEM images of MgO+Si+C composite at 700° C. for various holding times of 50 min, 100 min, 200 min, 400 min, and 600 min. These figures show that overall particle size does not change much but the morphology is significantly altered by the holding time from 50 min to 400 min. As shown in FIGS. 12A-12J, overall particle size did not appear to change much but morphology is significantly altered by a holding time from 50 min to 400 min. This morphology change is believed to be closely related to kinetics of the oxidation reaction, such as diffusion of CO.sub.2 into bulk Mg.sub.2Si and reaction rate of Mg.sub.2Si+CO.sub.2.fwdarw.2MgO+Si+C.

[0066] FIG. 13 shows example XRD patterns for CaO+Si+C composite after CO.sub.2-thermic oxidation process with CaSi.sub.2 (bottom) and Si+C after CaO removal (middle and top) in accordance with embodiments of the disclosed technology. As shown in FIG. 13, decrease in impurity phase (FeSi.sub.2) can be clearly seen after HF leaching.

EXPERIMENTS

[0067] Certain experiments of the CO.sub.2-thermic oxidation process using intermetallic compounds (Mg.sub.2Si and CaSi.sub.2) as model systems were performed, including a thermodynamics feasibility study, an in-situ phase evolution analysis, and intensive microstructural characterization of the CO.sub.2-thermic oxidation reaction.

[0068] The CO.sub.2-thermic oxidation process was found to be an effective way to fabricate suitable anode materials for advanced energy storage systems, such as lithium batteries. The reaction product showed excellent long-term cycling stability and moderate rate capability. These results showed that the proposed CO.sub.2-thermic oxidation process with oxidative intermetallic compounds is a pathway for recycling and utilizing CO.sub.2 gas as for the formation of functional energy materials.

Synthesis of Magnesium Silicide

[0069] Commercially available silicon metal powder was purchased. After manually mixing 1 gram of silicon powder and 2 grams of magnesium powder, the mixture was put into stainless steel autoclave reactor and then tightly sealed in inert environment. Excess magnesium than the stoichiometric ratio of Mg.sub.2Si was used to ensure full conversion of silicon to silicide. To reduce exposure to air, at least three times of inert gas purging was performed prior to sealing the reactor. After the sealing, the reactor was place in a tube furnace with heating condition set to 500° C. for 10 hours at 1° C./min thermal ramping rate, and then naturally cooled to room temperature.

CO.sub.2-Thermic Oxidation Process of Mg.sub.2Si

[0070] 2 grams of prepared magnesium silicide was grinded and spread in an alumina crucible. The crucible was then place in the tube furnace set to 700° C. for 10 hours at 1° C./min thermal ramping rate in carbon dioxide gas flow. After the heat treatment, the resultant powder was immersed in 1 mol/L hydrochloric acid for at least 2 hours to remove magnesium oxide and followed by vacuum filtering and drying at 60° C. overnight.

Materials Characterization

[0071] X-ray diffractometers, or XRD were used to investigate the crystalline structure and material composition. Powder morphology was characterized using field emission scanning electron microscope (FEI Quanta 200F) and transmission electron microscope (FEI Technai G2 20 Twin with 200 kV LaB6 electron source). Also, energy dispersive spectroscopy (Tescan Vega3) was used to perform elemental analysis. Raman spectroscopy (Horiba LabRAM HR system equipped with 532 nm laser) was used to investigate the structure of the carbon after in-situ synthesized silicon/carbon composite. In-situ XRD data were collected using Rigaku Smartlab equipped with Anton Paar XRK900S high temperature chamber. The XRD data were continuously recorded during heating and cooling. Each scan took approximately 30 minutes and the scan range (2θ) was from 20° to 50° where all the main peaks of silicon, magnesium silicide, magnesium oxide, and carbon can be detected.

Electrochemical Properties Measurements

[0072] To evaluate the electrochemical properties of the silicon/carbon composite, 2032 coin cells, having as-synthesized carbon-coated macro-porous silicon as working electrode and lithium metal chip (MTI) as counter electrode. The electrodes were prepared by slurry casting in which the slurry, composed of the carbon-coated macro-porous silicon material (60 wt %), Super P45 carbon black conducting agent (20 wt %), and poly(acrylic acid) (PAA, average molecular weight of 100 g/mol, Sigma Aldrich)/sodium carboxymethyl cellulose (CMC, 5 wt % in high purity water, Sigma Aldrich) binder (20 wt %, PAA:CMC=1:1 by weight) in purified distilled water, was evenly spread onto copper foil (current collector) and vacuum dried.

[0073] The electrolyte used was 1.3 mol/L lithium hexafluorophosphate (LiPF.sub.6) in ethylene carbonate (EC)/Diethylene carbonate (DEC) (3:7 by volume) with 10 wt % fluoroethylene carbonate (FEC) additive. Current condition for the battery cycling test was as follows; for the first activation cycle, 0.05 C-rate (very slow current) was applied to measure apparent capacity of each coin cell assembled. Based on the capacity from the first cycle (1 C-rate), subsequent cycling test was performed at 0.2 C-rate between voltage range of 0.01 V and 1.2 V. Based on the apparent capacity of each coin cell, current conditions for subsequent cycling test were recalculated (or adjusted) accordingly. The subsequent cycling test was performed at 0.2 C-rate between voltage range of 0.01 V and 1.2 V.

[0074] From the foregoing, it will be appreciated that specific embodiments of the disclosure have been described herein for purposes of illustration, but that various modifications may be made without deviating from the disclosure. In addition, many of the elements of one embodiment may be combined with other embodiments in addition to or in lieu of the elements of the other embodiments. Accordingly, the technology is not limited except as by the appended claims.