Methods for Producing Silicon-Containing Structures Using Redox Mediators and Chemical Reduction

Abstract

Described herein are methods for producing silicon-containing structures using electrochemically generated solutions and chemical reduction of components in such solutions. For example, a cathode solution and an anode solution may be provided a reactor with the cathode solution comprising a cathode solution solvent, a cathode solution salt, and a redox mediator and with the anode solution comprising an anode solution solvent and an anode solution salt. A voltage is then applied between the cathode and anode thereby converting the redox mediator into a reducing agent forming a charged cathode solution. The method may proceed with adding a silicon-containing precursor to the charged cathode solution such that the reducing agent reacts with the silicon-containing precursor and forms silicon-containing structures and a precursor-mixture salt in the precursor mixture. The redox mediator is released into the precursor mixture during this operation. The method proceeds with separating the silicon-containing structures from the precursor mixture.

Claims

1. A method comprising: providing a cathode solution and an anode solution in a reactor comprising a cathode, an anode, and a separator, wherein: the cathode solution comprises a cathode solution solvent, a cathode solution salt, and a redox mediator, the anode solution comprises an anode solution solvent and an anode solution salt, and both the cathode solution and anode solution comprise charge-carrying ions; applying a voltage between the cathode and the anode thereby converting the cathode solution into a charged cathode solution comprising a reducing agent formed from the redox mediator by adding electrons received from the cathode; adding a silicon-containing precursor to the charged cathode solution thereby forming a precursor mixture, wherein the reducing agent reacts with the silicon-containing precursor and forms silicon-containing structures and a precursor-mixture salt in the precursor mixture while releasing the redox mediator into the precursor mixture; and separating the silicon-containing structures from the precursor mixture.

2. The method of claim 1, wherein the reducing agent comprises one or more solvated electrons, a reduced form of the redox mediator, metal hydrides, or reduced metal ions.

3. The method of claim 1, wherein the precursor-mixture salt is same as the cathode solution salt.

4. The method of claim 1, wherein the charge-carrying ions is one or more cations selected from the group consisting of H.sup.+, Li.sup.+, Na.sup.+, K.sup.+, Cs.sup.+, tetramethylammonium cation (TMA.sup.+), tetraethylammonium cation (TEA.sup.+), tetrapentylammonium cation (TPA.sup.+), tetrabutylammonium cation (TBA.sup.+), and 1-butyl-1-methylpyrrolidinium cation (PYR.sub.14.sup.+).

5. The method of claim 1, wherein the charge-carrying ions is one or more anions selected from the group consisting of F.sup., Cl.sup., Br, I.sup., perchlorate anion (ClO.sub.4.sup.), nitrate anion (NO.sub.3.sup.), hexafluorophosphate anion (PF.sub.6.sup.), silicon pentachloride anion (SiCl.sub.5.sup.), bis(fluorosulfonyl)imide anion (FSI.sup.), and bis(trifluoromethylsulfonyl)imide (TFSI.sup.).

6. The method of claim 1, further comprising heat-treating the silicon-containing structures separated from the precursor mixture.

7. The method of claim 1, further comprising: after separating the silicon-containing structures from the precursor mixture, reusing the precursor mixture as the cathode solution or anode solution, and repeating applying the voltage, adding the silicon-containing precursor, and separating the silicon-containing structures from the precursor mixture.

8. The method of claim 7, wherein separating the silicon-containing structures from the precursor mixture further comprises removing the precursor-mixture salt from the precursor mixture thereby forming the cathode solution.

9. The method of claim 8, wherein separating the silicon-containing structures from the precursor mixture further comprises: extracting a solid mixture from the precursor mixture; adding a salt-dissolving solvent to dissolve the precursor-mixture salt formed in the solid mixture, forming a slurry mixture; separating the slurry mixture into the silicon-containing structures and a salt solution; and removing the salt-dissolving solvent from the salt solution, prior to reusing the precursor-mixture salt as the cathode solution salt or anode solution salt.

10. The method of claim 9, wherein: the cathode solution solvent is diglyme, the cathode solution salt is a sodium perchlorate (NaClO.sub.4), the redox mediator is naphthalene (C.sub.10H.sub.8), the separator is NASCION-type solid electrolyte Na.sub.3Zr.sub.2Si.sub.2PO.sub.12 (NZSP), the charge-carrying ions are sodium ions (Na.sup.+), the anode solution solvent is water, the anode solution salt is sodium chloride, the voltage is 5V, the silicon-containing precursor is silicon tetrachloride (SiCl.sub.4), and the salt-dissolving solvent is water.

11. The method of claim 1, wherein separating the silicon-containing structures from the precursor mixture further comprises extracting and recycling the precursor-mixture salt from the precursor mixture thereby forming the cathode solution salt or anode solution salt.

12. The method of claim 1, wherein applying a voltage between the cathode and the anode comprises: determining an equivalent charge of the charged cathode solution, and determining an amount of the silicon-containing precursor added to the charged cathode solution based on the equivalent charge of the charged cathode solution.

13. The method of claim 1, wherein: the cathode solution solvent is tetrahydrofuran (THF), the cathode solution salt is a lithium hexafluorophosphate (LiPF.sub.6), the redox mediator is biphenyl (C.sub.12H.sub.10), the voltage is 3V, and the silicon-containing precursor is silicon tetrachloride (SiCl.sub.4).

14. The method of claim 1, wherein separating the silicon-containing structures from the precursor mixture is performed using a centrifuge or filtration.

15. The method of claim 1, wherein at least one of the cathode and the anode comprises a metal, a carbon, a conductive polymer, a conductive ceramic, or a conductive silicon.

16. The method of claim 1, wherein the cathode solution solvent is one of: an ether selected from the group consisting of tetrahydrofuran (THF), monoglyme, diglyme, triglyme, and tetraglyme, an organic carbonate selected from the group consisting of propylene carbonate (PC), and dimethyl carbonate (DMC), and an ionic liquid selected from the group consisting of 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI), N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI), 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI), 1-butyl-3-methylimidazolium tetrachloroaluminate ([BMIM]AlCl.sub.4), and acetonitrile (C.sub.2H.sub.3N).

17. The method of claim 1, wherein the cathode solution salt is selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl.sub.2), aluminum chloride (AlCl.sub.3), tetramethylammonium chloride (TMACl), tetraethylammonium chloride (TEACl), tetrapentylammonium chloride (TPACl), tetrabutylammonium chloride (TBACl), tetrabutylammonium bromide (TBABr), N-butyl-N-methylpyrrolidinium chloride (PYR.sub.14Cl), N-methyl-N-propylpyrrolidinium chloride (PYR13Cl), lithium hexafluorophosphate (LiPF.sub.6), sodium hexafluorophosphate (NaPF.sub.6), lithium perchlorate (LiClO.sub.4), sodium perchlorate (NaClO.sub.4), lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), sodium bis(trifluoromethane)sulfonimide (NaTFSI), and NaFSI.

18. The method of claim 1, wherein the redox mediator is selected from the group consisting of biphenyl (C.sub.12H.sub.10), naphthalene (C.sub.10H.sub.8), methylamine (CH.sub.3NH.sub.2), a crown ether, metallocene, cobaltocene (Co(.sub.5C.sub.5H.sub.5).sub.2]), decamethylcobaltocene (C.sub.20H.sub.30Co), ferrocene, decamethylferrocene, chromocene, nickelocene, metal carbonyl, nickel tetracarbonyl, iron pentacarbonyl, chromium hexacarbonyl, and dimanganese decacarbonyl.

19. The method of claim 1, wherein the separator comprises one or more materials selected from the group consisting of a dense solid electrolyte, dense or porous ion-selective membrane, porous polymer, porous glass, and porous ceramic.

20. The method of claim 1, wherein the separator is one of: an ion-selective membrane selected from the group consisting of NASCION-structured Na.sub.3Zr.sub.2Si.sub.2PO.sub.12, lithium aluminum titanium phosphate (LATP), Garnet-type lithium lanthanum zirconium oxide (LLZO), and ABO3-type Lithium niobate (LiNbO.sub.3), a cation exchange polymer electrolyte selected from the group consisting of NAFION, polyether ether ketone (PEEK), and polyethylene oxide (PEO), and anion exchange polymer electrolyte selected from the group consisting of polymeric quaternary ammonium chloride and bromide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The included drawings are for illustrative purposes and serve only to provide examples of possible structures and operations for the disclosed inventive systems, apparatus, and methods. These drawings in no way limit any changes in form and detail that may be made by one skilled in the art without departing from the spirit and scope of the disclosed implementations.

[0036] FIG. 1 is a process flowchart of a method for the low-temperature electrochemical deposition of active materials for electrochemical cells using electrochemically-generated solutions and chemical reduction of components, in accordance with some examples.

[0037] FIG. 2A is a schematic block diagram of a reactor with a cathode solution and anode solution used to generate a charged cathode solution comprising a reducing agent formed from the redox mediator, in accordance with some examples.

[0038] FIG. 2B is a schematic block diagram of forming a charged cathode solution in the reactor of FIG. 2A, in accordance with some examples.

[0039] FIG. 2C is a schematic block diagram of using a charged cathode solution to generate silicon-containing structures.

[0040] FIG. 3A is a CV plot of biphenyl (C.sub.12H.sub.10) reduction and oxidation in an electrolyte containing lithium hexafluorophosphate (LiPF.sub.6) and tetrahydrofuran (THF).

[0041] FIG. 3B illustrates an X-ray diffraction (XRD) of amorphous silicon harvested after silicon tetrachloride (SiCl.sub.4) was added to the charge cathode solution (formed by biphenyl (C.sub.12H.sub.10) reduction).

DETAILED DESCRIPTION

[0042] In the foregoing specification, various techniques and mechanisms may have been described in singular form for clarity. However, it should be noted that some embodiments include multiple iterations of a technique or multiple instantiations of a mechanism unless otherwise noted. For example, a system uses a processor in a variety of contexts but can use multiple processors while remaining within the scope of the present disclosure unless otherwise noted. Similarly, various techniques and mechanisms may have been described as including a connection between two entities. However, a connection does not necessarily mean a direct, unimpeded connection, as a variety of other entities (e.g., bridges, controllers, gateways, etc.) may reside between the two entities.

INTRODUCTION

[0043] As noted above, conventional synthesis of silicon structures typically requires high processing temperatures, e.g., using silicon halides with Siemens method. For example, temperatures can reach or exceed 500 C. or even 1000 C. Such temperatures involve high energy costs and produce silicon particles with large grain sizes and high crystallinity (such as grain size over 10 nanometers and 100% crystalline). Some of these characteristics are not suitable for negative active materials in lithium-ion batteries due to dramatic atomic re-ordering during the crystalline to amorphous transition upon lithiation. Crystalline silicon with a large grain size is more prone to crack than amorphous silicon with a small domain size.

[0044] Low-temperature synthesis of silicon structures is very desirable for fabricating structures having amorphous phases (e.g., the crystallinity of less than 90% or even less than 50% or tunable crystallinity) silicon and small grain sizes (e.g., less than 10 nanometers or even less than 1 nanometer). For purposes of this disclosure, the low temperature is defined as a process performed at a temperature of less than 200 C. or even less than 100 C.

[0045] Described herein are methods and systems that utilize electrochemical reduction to generate/regenerate redox mediators that can be later used to reduce silicon halides into silicon (to form silicon structures with desired characteristics). The process can be a closed loop that continuously generates silicon structures of desirable grain sizes and crystallinity. Specifically, an electrochemical process is used to convert a redox mediator into its reduced form, which may be referred to as a reducing agent. This reducing agent can be a liquid state of its own or as a solute in a solution. This reducing agent can also be a gas that is dissolved in the liquid solution or as a solid solution stored in a solid that can be released in control. This freshly formed reducing agent is capable and used to reduce the silicon halides into silicon, while the reducing agent also converts back to its original oxidized form (and reused in a new production cycle).

[0046] Using the electrochemical processing of the redox mediator has various advantages over the direct chemical reduction of silicon halides by alkaline and alkaline earth metals. First, electrochemical processing does not require the continuous input of alkaline and alkaline earth metals, which can be costly. Second, electrochemical processing does not have the passivation effect from the solid silicon structures formed at the solid (metal)liquid (silicon halide) interface. When using solid reducing agent, such as alkaline and alkaline earth metals, the formation of the solid silicon structures will fully cover the solid reducing agents, blocking the contact between the solid (metal) and liquid (silicon halide), therefore limiting the continuous reaction for silicon to form. Instead, the reaction between the liquid-reducing agent and silicon halide is a liquid-liquid reaction that produces solid silicon structures via homogenous nucleation. This homogenous nucleation process does not rely on the interface reaction to proceed. Therefore, the reaction can be rapid and continuous. Third, a lower deposition temperature allows forming silicon structures in an amorphous phase with small particle sizes. High-temperature processing will inevitably induce amorphous to crystalline transition and the growth of grain size together with particle size. Both the grain size and particle size can influence the stability of the silicon during the repeated volume expansion and shrinking during the cycling as the anode/negative electrode of lithium ion batteries. Amorphous silicon with smaller grain size and particle size is known to be more desirable.

Examples of Forming Silicon-Containing Structures

[0047] FIG. 1 is a process flowchart corresponding to method 100 for forming silicon-containing structures 251. Method 100 may also be referred to as a low-temperature deposition method and/or liquid-based deposition method as the formation of silicon-containing structures 251 is performed at low temperatures (e.g., less than 200 C., less than 100 C.) and in liquid phases.

[0048] In some examples, method 100 comprises (block 110) providing a cathode solution 210 and an anode solution 220 in an reactor 200, e.g., as schematically shown in FIG. 2A. The reactor 200 comprises a cathode 201, an anode 202, and a separator 203. Various examples of the reactor 200 are within the scope. In some examples, the cathode 201 comprises a metal, a carbon, a conductive polymer, a conductive ceramic, or a conductive silicon that are stable toward the reduction potential applied here. The anode 202 may comprise a material selected from the group consisting of a metal, a carbon, a conductive polymer, a conductive ceramic, or a conductive silicon that is stable toward the oxidation potential applied here. The cathode 201 and the anode 202 can be connected to the power supply to apply a desired voltage between the cathode 201 and the anode 202.

[0049] In some examples, the separator 203 of the reactor 200 comprises one or more materials selected from the group consisting of a dense solid electrolyte, dense or porous ion-selective membrane, porous polymer, porous glass, and porous ceramic. For example, the separator 203 may be one of (a) a dense solid electrolyte selected from the group consisting of NASCION-structured Na.sub.3Zr.sub.2Si.sub.2PO.sub.12 (NZSP), lithium aluminum titanium phosphate (LATP), Garnet-type lithium lanthanum zirconium oxide (LLZO), ABO3-type lithium niobate (LiNbO.sub.3), (b) a cation exchange polymer electrolyte selected from the group consisting of NAFION, polyether ether ketone (PEEK), and polyethylene oxide (PEO), and (c) anion exchange polymer electrolyte selected from the group consisting of polymeric quaternary ammonium chloride, bromide, iodide or hydroxide. In another example, the separator 203 may be a porous separator with an average pore size below 10 micrometers, 1 micrometer, 100 nanometers, 10 nanometers, 1 nanometer, or 0.5 nanometers. The smaller pore size will help to limit the diffusion of electrolyte components other than the charge-carrying ions through the size screening effect. In some examples, the pores of the porous separator can be further surface modified to induce a charging screening effect. The separator 203 is specially selected to allow the charge-carrying ions 226 through the separator 203 without allowing other components through.

[0050] The cathode solution 210 comprises a cathode solution solvent 212, a cathode solution salt 214, and a redox mediator 216. In some examples, the cathode solution solvent 212 is selected from the group consisting of an ether (e.g., tetrahydrofuran (THF), monoglyme, diglyme, triglyme, tetraglyme), an organic carbonate (e.g., propylene carbonate (PC), dimethyl carbonate (DMC), an ionic liquid (e.g., 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI), N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI), 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI), 1-butyl-3-methylimidazolium tetrachloroaluminate ([BMIM]AlCl4), and acetonitrile (C.sub.2H.sub.3N). The solvent should provide sufficient solubility to both the salts and redox mediators while possessing a wide enough electrochemical and chemical stability window to enable the electrochemical reaction. For example, ether is more stable toward reductive potential and can be used.

[0051] In some examples, the cathode solution salt 214 is selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl.sub.2), aluminum chloride (AlCl.sub.3), tetramethylammonium chloride (TMACl), tetraethylammonium chloride (TEACl), tetrapentylammonium chloride (TPACl), tetrabutylammonium chloride (TBACl), tetrabutylammonium bromide (TBABr), N-butyl-N-methylpyrrolidinium chloride (PYR14Cl), N-methyl-N-propylpyrrolidinium chloride (PYR13Cl), lithium hexafluorophosphate (LiPF.sub.6), sodium hexafluorophosphate (NaPF.sub.6), lithium perchlorate (LiClO.sub.4), sodium perchlorate (NaClO.sub.4), lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), sodium bis(trifluoromethane)sulfonimide (NaTFSI), sodium bis(fluorosulfonyl)imide (NaFSI), and the like. The salt should have enough solubility in the solvent and provide high conductivity in the solution. The salt can also possess different solubility in different solvents to allow it to be recycled. In some examples, the concentration of the cathode solution salt 214 in the cathode solution 210 is between 0.05 mol/L and 2 mol/L or, more specifically, between 0.1 mol/L and 1 mol/L.

[0052] In some examples, the redox mediator 216 is selected from the group consisting of biphenyl (C.sub.12H.sub.10), naphthalene (C.sub.10H.sub.8), methylamine (CH.sub.3NH.sub.2), a crown ether (such as 8-crown-4,18-crown-6, dibenzo-18-crown-6), metallocene ((C.sub.5H.sub.5).sub.2M) such as cobaltocene (Co(.sub.5C.sub.5H.sub.5).sub.2]), decamethylcobaltocene (C.sub.20H.sub.30Co), ferrocene, decamethylferrocene, chromocene, nickelocene, metal carbonyl, nickel tetracarbonyl, iron pentacarbonyl, chromium hexacarbonyl, dimanganese decacarbonyl, and hydrides. The redox mediator should have a reduced state that is reducing enough to react with silicon precursor to form solid silicon. Such a reduced state should be attainable via an electrochemical reduction on the electrode and should be stable in the cathode solution for a reasonable time before the reaction to the silicon precursors. In some examples, the concentration of the redox mediator 216 in the cathode solution 210 is between 0.1 mol/L to 10 mol/L or, more specifically, 0.5 mol/L to 5 mol/L, 1 mol/L to 2 mol/L, and 2 mol/L to 4 mol/L.

[0053] In some examples, the redox mediator 216 is selected from simple metal cations that may be reduced to a soluble reduced cation with lower oxidation state as the reducing agent. For example, trivalent neodymium cation (Nd.sup.3+) can be reduced to bivalent neodymium cation (Nd.sup.2+) at the potential of 2.7 V vs standard hydrogen electrode (SHE), which is sufficient to reduce silicon halides. Other examples include tetravalent thorium cation (Th.sup.4+), Th.sup.4+.Math.Th.sup.3+; trivalent praseodymium cation (Pr.sup.3+), Pr.sup.3+.Math.Pr.sup.2+; trivalent erbium cation (Er.sup.3+), Er.sup.3+.Math.Er.sup.2+; trivalent promethium cation (Pm.sup.3+), Pm.sup.3+.Math.Pm.sup.2+; and trivalent dysprosium cation (Dy.sup.3+), Dy.sup.3+.Math.Dy.sup.2+. The metal cation may be selected based on, for example, solubility of a salt comprising the cation in the cathode solution solvent 212, or compatibility of the cation with the cathode solution solvent 212.

[0054] In some examples, the redox mediator 216 is selected from simple metal cations that may be reduced to form a solid solution as the reducing agent. For example, lithium ions can be reduced and alloy with graphite forming LiC.sub.6 at2.84V vs SHE, which is sufficient to reduce silicon halides as a solid solution reducing agent. Other examples may include LiSi alloy, MgSi alloy, PdH alloy, etc. The metal cation may be selected based on, for example, solubility of a salt comprising the cation in the cathode solution solvent 212, the reduction potential of the cation, or compatibility of the cation with the cathode solution solvent 212.

[0055] In some examples, the redox mediator 216 is selected from simple metal cations that may be reduced to form solid materials as the reducing agent. For example, the solid materials can be lithium metal from lithium ions, sodium metal from sodium ions, magnesium metal from magnesium ions. Other examples include sodium metal, magnesium metal, potassium metal, rubidium metal, cesium metal, calcium metal, strontium metal, lanthanum metal, yttrium metal, praseodymium metal, and cerium metal. The metal cation may be selected based on, for example, solubility of a salt comprising the cation in the cathode solution solvent 212, the reduction potential of the cation, or compatibility of the cation with the cathode solution solvent 212.

[0056] The anode solution 220 comprises an anode solution solvent 222 and an anode solution salt 224. In some examples, the anode solution solvent 222 is selected from the group consisting of an ether (e.g., tetrahydrofuran (THF), monoglyme, diglyme, triglyme, tetraglyme), an organic carbonate (e.g., propylene carbonate (PC), dimethyl carbonate (DMC), an ionic liquid (e.g., 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI), N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI), 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI), 1-butyl-3-methylimidazolium tetrachloroaluminate ([BMIM]AlCl4), acetonitrile (C.sub.2H.sub.3N), an amide, an ester, a sulfone, a sulfoxide, and water. In some examples, the anode solution salt 224 is selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl.sub.2), aluminum chloride (AlCl.sub.3), tetramethylammonium chloride (TMACl), tetraethylammonium chloride (TEACl), tetrapentylammonium chloride (TPACl), tetrabutylammonium chloride (TBACl), tetrabutylammonium bromide (TBABr), N-butyl-N-methylpyrrolidinium chloride (PYR14Cl), N-methyl-N-propylpyrrolidinium chloride (PYR13Cl), lithium hexafluorophosphate (LiPF.sub.6), sodium hexafluorophosphate (NaPF.sub.6), lithium perchlorate (LiClO.sub.4), sodium perchlorate (NaClO.sub.4), lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), sodium bis(trifluoromethane)sulfonimide (NaTFSI), sodium bis(fluorosulfonyl)imide (NaFSI). In some examples, the concentration of the anode solution salt 224 in the anode solution 220 is between 0.05 mol/L and 2 mol/L or, more specifically, is between 0.1 mol/L and 1 mol/L.

[0057] The anode solution salt 224 produces charge-carrying ions 226, which can migrate to the cathode solution 210. In some examples, the charge-carrying ions 226 are one or more cations selected from the group consisting of H.sup.+, Li.sup.+, Na.sup.+, K.sup.+, Cs.sup.+, tetramethylammonium cation (TMA.sup.+), tetraethylammonium cation (TEA.sup.+), tetrapentylammonium cation (TPA+), tetrabutylammonium cation (TBA.sup.+), and 1-butyl-1-methylpyrrolidinium cation (PYR.sub.14.sup.+). Cations are usually small in ionic diameter. In particular, these monovalent cations are easy to transport through the separator 203, providing high conductivity.

[0058] In some examples, the charge-carrying ions 226 are one or more anions selected from the group consisting of fluoride (F.sup.), chloride (Cl.sup.), bromide (Br.sup.), iodide (I.sup.), perchlorate anion (ClO.sub.4.sup.), nitrate anion (NO.sub.3.sup.), hexafluorophosphate anion (PF.sub.6.sup.), silicon pentachloride anion (SiCl.sub.5.sup.), bis(fluorosulfonyl)imide anion (FSI.sup.), and bis(trifluoromethylsulfonyl)imide (TFSI.sup.) These anions can be combined with proper cations to provide sufficient solubility and conductivity to allow charge transfer between the two reactor chambers. When ion-selective membranes are selected as separator 203, the charge-carrying ion 226 must be the same or compatible with the mobile charge species of separator 203. For example, when a cation exchange membrane of Nafion is used as separator 203, the charge-carrying ion 226 must be a cation such as Na.sup.+, Li.sup.+, or H.sup.+ rather than an anion such as Cl. In another example, when an anion exchange membrane of polymeric quaternary ammonium chloride is used as separator 203, the charge-carrying ion 226 must be an anion such as Cl.sup., Br.sup., or OH.sup., rather than a cation such as Na.sup.+, Li.sup.+, or H.sup.+.

[0059] Method 100 also comprises (block 120) applying a voltage between the cathode 201 and the anode 202 thereby converting the cathode solution 210 into a charged cathode solution 230 comprising reducing agent 236 formed from the redox mediator 216 by adding electrons received from the cathode 201, e.g., as schematically shown in FIG. 2B. In some examples, the voltage is 0-6V or, more specifically, 0-1V, 1-5V, 1-4V, 1-3V, 1-2V, 2-5V, 2-4V, 2-3V, 3-5V, 3-4V, 4-5V. The voltage should be high enough in magnitude to induce the reductive reaction of the redox mediator while not too high to induce electrolyte decomposition. The specific value depends on the combination of redox mediator selection and the counter-electrode reaction. The minimal voltage is the potential difference between the redox mediator reduction potential and the counter-electrode oxidation reaction potential.

[0060] In some examples, the reducing agent 236 comprises one or more solvated electrons, which is a reduced form of redox mediator 216, metal hydrides, and reduced metal ions. Specific examples include, but are not limited to the following reactions:

##STR00001##

[0061] In some examples, (block 120) applying a voltage between the cathode 201 and the anode 202 comprises (block 122) determining an equivalent charge of the charged cathode solution 230 and also comprises (block 124) determining an amount of the silicon-containing precursor 240 added to the charged cathode solution 230 based on the equivalent charge of the charged cathode solution. In one example, with the input of 1 Ampere current for 1 h, 1 Ah charge was provided to reduce redox mediator 216 to reducing agent 236, by assuming 100% Faradaic efficiency, this means the formed reducing agent holds 3600 Coulombs of charge that is available to reduce silicon-containing precursor 240. If the silicon oxidation state in the silicon-containing precursor 240 is 4.sup.+, it can reduce to produce 0.00932 mol of silicon to the oxidation state of 0. This value determines the quantity of silicon precursor that can be added to the charged cathode solution 230.

[0062] In some examples, the anions of the anode solution salt 224 form a gas (e.g., a chlorine gas) that is removed from the reactor 200.

[0063] The method 100 also comprises (block 130) adding a silicon-containing precursor 240 to the charged cathode solution 230 thereby forming a precursor mixture 250 as, e.g., is schematically shown in FIG. 2C. Specifically, once the silicon-containing precursor 240 is added, the reducing agent 236 reacts with the silicon-containing precursor 240 and forms silicon-containing structures 251 and a precursor-mixture salt 254 in the precursor mixture 250. This reaction also corresponds to releasing the redox mediator 216 into the precursor mixture 250.

[0064] In some examples, the silicon-containing precursor 240 is selected from the group consisting of silicon tetrachloride (SiCl.sub.4), hexachlorodisilane (Si.sub.2Cl.sub.6), silicon tetrabromide (SiBr.sub.4), silicon tetraiodide (SiI.sub.4), trichlorosilane (HSiCl.sub.3), CH.sub.3SiCl.sub.4, (CH.sub.3).sub.2SiCl.sub.2, (CH.sub.3).sub.3SiCl, H(CH.sub.3)SiCl.sub.2, and Ph-SiCl.sub.3. As noted above, the amount of the silicon-containing precursor 240 may be determined by the equivalent charge of the charged cathode solution 230.

[0065] The above process can be illustrated with specific examples. In one example, the cathode solution solvent 212 is diglyme. The cathode solution salt 214 is a sodium perchlorate (NaClO.sub.4). The redox mediator 216 is naphthalene (C.sub.10H.sub.8). The separator 203 is a NASCION-type solid electrolyte Na.sub.3Zr.sub.2Si.sub.2PO.sub.12 (NZSP). The charge-carrying ions 226 are sodium ions (Na.sup.+). The anode solution solvent 222 is water. The anode solution salt 224 is sodium chloride. The voltage is 5V and is applied for 4 hours. The silicon-containing precursor 240 is silicon tetrachloride (SiCl.sub.4). The salt-dissolving solvent 252 is water. [0066] Cl oxidation on the anode: 2Cl.sup.2e=Cl.sub.2 [0067] naphthalene (C.sub.10H.sub.8) reduction on cathode: C.sub.10H.sub.8+e=C.sub.10H.sub.8.sup. [0068] silicon reduction by reduced naphthalene (C.sub.10H.sub.8): 4 C.sub.10H.sub.8.sup.+SiCl.sub.4=Si+4 C.sub.10H.sub.8+4Cl.sup. [0069] Na.sup.+ is balancing the charge [0070] The net reaction is SiCl.sub.4=Si+2Cl.sub.2

[0071] In another example, the cathode solution solvent 212 is tetrahydrofuran (THF). The cathode solution salt 214 is a lithium hexafluorophosphate (LiPF.sub.6). The redox mediator 216 is biphenyl (C.sub.12H.sub.10). The voltage is 3V and is applied for 4 hours. The silicon-containing precursor 240 is silicon tetrachloride (SiCl.sub.4). The charge-carrying ions 226 in this example are lithium ions (Li.sup.+). As the charge-carrying ions 226 are accumulated into the recycled cathode solution, these cations form LiBP and precipitate out as lithium chloride (LiCl) after mixing with silicon tetrachloride (SiCl.sub.4). lithium chloride (LiCl) can be recycled. [0072] Cl.sup. oxidation on the anode: 2Cl.sup.2e=Cl.sub.2 [0073] biphenyl (C.sub.12H.sub.10) reduction on cathode: C.sub.12H.sub.10+2e=C.sub.12H.sub.10.sup.2 [0074] silicon reduction by reduced biphenyl (C.sub.12H.sub.10): 2 C.sub.12H.sub.10.sup.2-+SiCl.sub.4=Si+2C.sub.12H.sub.10+4Cl.sup. [0075] Li.sup.+ is balancing the charge [0076] The net reaction is SiCl.sub.4=Si+2Cl.sub.2

[0077] In some examples, the precursor-mixture salt 254 is the same as the cathode solution salt 214. Specifically, the cathode solution salt 214 is needed for ionic conductivity within the cathode solution 210 while forming the charged cathode solution 230 (as described above). In some examples, the salt formation in the precursor mixture 250 is beneficial to create a porous structure of the silicon structure. The salt initially occupies some volume within the formed silicon structure and can free up this volume as embedded pores after proper solvent is used to dissolve the salt. These pores can be helpful to reduce the volume expansion during the lithiation process of the silicon.

[0078] The method 100 also comprises (block 140) separating the silicon-containing structures 251 from the precursor mixture 250, e.g., a centrifuge or filtration.

[0079] In more specific examples, (block 140) separating the silicon-containing structures 251 from the precursor mixture 250 further comprises (block 146) removing the precursor-mixture salt 254 from the precursor mixture 250 thereby forming the cathode solution 210. For example, the method 100 may further comprise (block 150) reusing the precursor mixture 250 as the cathode solution 210 (which is performed after (block 140) separating the silicon-containing structures 251 from the precursor mixture 250). Various operations of method 100 (e.g., applying the voltage, adding the silicon-containing precursor 240, and separating the silicon-containing structures 251 from the precursor mixture 250) may be repeated with the reused precursor mixture 250.

[0080] In some examples, (block 140) separating the silicon-containing structures 251 from the precursor mixture 250 further comprises (block 148) extracting the precursor-mixture salt 254 from the precursor mixture 250 thereby being re-used as the cathode solution salt 214 or anode solution salt 224. This extraction may be used for recycling the precursor-mixture salt 254. In further examples, extracting the precursor-mixture salt 254 from the precursor mixture 250 further comprises extracting a solid mixture 255 from the precursor mixture 250; (block 142) adding a salt-dissolving solvent 252 to dissolve the precursor-mixture salt 254 from the solid mixture 255, forming a slurry mixture 256; (block 144) separating the slurry mixture 256 into the silicon-containing structures 251 and salt solution 257; and (block 146) removing the salt-dissolving solvent 252 from the salt solution 257, prior to re-using the precursor-mixture salt as the cathode solution salt 214 or anode solution salt 224.

[0081] In some further examples, a salt-dissolution promoter 253 may be added together with the salt-dissolving solvent 252 to dissolve the precursor-mixture salt 254 from the solid mixture 255, forming a slurry mixture 256. The salt-dissolution promoter 253 may be a Lewis acid or base, depending on the nature of the salt to be dissolved. For example, 18-6 crown ether was known to coordinate with K.sup.+ which can promote the dissolution of potassium chloride (KCl). Chelating agents, such as ethylenediaminetetraacetic acid (EDTA), citric acid, ethylenediamine, and porphine, can promote the dissolution of Ca.sup.2+, Mg.sup.2+, Ni.sup.2+, and Fe.sup.2+ containing salts. In other examples, boron trichloride (BCl.sub.3), aluminum chloride (AlCl.sub.3), gallium chloride (GaCl.sub.3), lanthanum chloride (LaCl.sub.3), titanium chloride (TiCl.sub.4), silicon tetrachloride (SiCl.sub.4), phosphorus trichloride (PCl.sub.3), phosphorus pentachloride (PCl.sub.5) can promote the dissolution of certain other chloride salts with bulky or bivalent cations, such as N-butyl-N-methylpyrrolidinium chloride (PYR.sub.14Cl), 1-Ethyl-3-methylimidazolium chloride, tetrabutylammonium chloride (TBACl), CsCl, magnesium chloride (MgCl.sub.2), calcium chloride (CaCl.sub.2), potassium chloride (KCl), lithium chloride (LiCl), or sodium chloride (NaCl). Similarly, the salt-dissolution promoter 253 may be other halides, such as fluorides, bromides or iodides with the cations K.sup.+, Ca.sup.2+, Mg.sup.2+, Ni.sup.2+, and Fe.sup.2+. The salts to be removed may also be other halides, such as fluorides, bromides or iodides with the cations K.sup.+, Ca.sup.2+, Mg.sup.2+, Ni.sup.2+, and Fe.sup.2+.

[0082] In the specific example described above (i.e., Cl oxidation on the anode: 2Cl.sup.2e=Cl.sub.2; naphthalene (C.sub.10H.sub.8) reduction on cathode: C.sub.10H.sub.8+e=C.sub.10H.sub.8.sup.; silicon reduction by reduced naphthalene 4 C.sub.10H.sub.8.sup.+SiCl.sub.4=Si+4 C.sub.10H.sub.8+4Cl.sup.; Na.sup.+ is balancing the charge; with the net reaction being SiCl.sub.4=Si+2Cl.sub.2), the charged cathode solution comprises diglyme (cathode solution solvent 212), sodium perchlorate (NaClO.sub.4) (cathode solution salt 214), and NaC.sub.10H.sub.8(Reducing agent 236) which reacts with silicon tetrachloride (SiCl.sub.4) (SiIicon-containing precursor 240) to form sodium chloride (NaCl) (precursor-mixture salt 254) and Si (silicon-containing structures 251) that are both not soluble in cathode solution solvent 212. Therefore, after solid-liquid separation by centrifuge or filtration, the solid mixture 255 comprises both NaCl (precursor-mixture salt 254) and Si (silicon-containing structures 251). A sodium chloride (NaCl)-dissolving solvent (salt-dissolving solvent 252), such as water, can be added to this solid mixture 255 to dissolve the sodium chloride (NaCl) (precursor-mixture salt 254) into the liquid phase (salt solution 257) and separate from silicon-containing structures 251 which is still a solid. The sodium chloride (NaCl) (precursor-mixture salt 254) in water (salt-dissolving solvent 252) solution (salt solution 257) can be re-used as anode solution 220 directly. Alternatively, the water (salt-dissolving solvent 252) of this salt solution 257 can be evaporated to be removed, leaving only sodium chloride (NaCl) (precursor-mixture salt 254) to be used as anode solution salt 224.

[0083] In some examples, method 100 further comprises (block 160) heat-treating the silicon-containing structures 251 separated from the precursor mixture 250. Further heat treatment can tune the domain size, particle size, and crystallinity of silicon-containing structures 251 to the desired values. The silicon-containing structures 251 produced from this low-temperature deposition feature a fully amorphous (100%) structure and small domain size (less than even 0.2 nanometers). Upon heating to above 300 degrees the domain will grow to above 0.3 nanometers, 0.5 nanometers, or even 1 nanometer, but without the formation of a crystalline phase. A smaller grain size is good for the cycling stability of the silicon but a larger grain size is good for the Coulombic efficiency. This tunable property allows the optimization of overall performance of the silicon anode.

Experimental Results

[0084] A series of experiments were conducted to produce silicon-containing structures using various examples of methods and systems described above. In one experiment, 0.5M of lithium hexafluorophosphate (LiPF.sub.6) as the salt and 1M biphenyl (C.sub.12H.sub.10) as the redox mediator can be dissolved in tetrahydrofuran (THF) solution as an electrolyte. Lithium aluminum titanium phosphate (LATP) solid electrolyte can be used as the membrane to separate the cathode solution and anode solution. The electrolysis can take place between the cathode and anode at 3V for 4 hours, resulting in the cathode solution being converted into a dark blue state. Biphenyl (C.sub.12H.sub.10) reduction to biphenyl lithium on cathode: Li.sup.++C.sub.12H.sub.10+2e=Li.sub.2C.sub.12H.sub.10. The results are shown in FIG. 3A, which is a CV plot of biphenyl (C.sub.12H.sub.10) reduction and oxidation in the electrolyte containing lithium hexafluorophosphate (LiPF.sub.6) and tetrahydrofuran (THF). The reduction potential of biphenyl (C.sub.12H.sub.10) is around +0.33V vs Li.sup.+/Li and reversible, which indicates that the reducing powder of reduced biphenyl (C.sub.12H.sub.10) is enough to reduce silicon tetrachloride (SiCl.sub.4) to Si (about +2.0V vs Li.sup.+/Li)

[0085] The cathode solution has been transferred to another container, for the addition and reaction with silicon tetrachloride (SiCl.sub.4). The amount of silicon tetrachloride (SiCl.sub.4) was determined by the equivalent charge as the electrolysis process, i.e., n(SiCl.sub.4)=Q/4/96485. silicon reduction by biphenyl lithium 2 Li.sub.2C.sub.12H.sub.10+SiCl.sub.4=Si+2 C.sub.12H.sub.10+4 LiCl.

[0086] Solid silicon structures were formed upon the addition of silicon tetrachloride (SiCl.sub.4) and were separated from the solution via centrifuge. The silicon product was further cleaned and heat-treated to get the desired crystallinity. FIG. 3B illustrates an XRD of amorphous silicon harvested during this operation. The broad diffraction peak at around 29 represents the (111) planes of silicon, and another broad peak also presents at around 52. These two broadened peaks together are characteristic of the amorphous state of silicon. The absence of sharp peaks at 28.40, 47.30 and 56.1 further corroborates the amorphous nature. The remaining solution was recycled to the reactor for the next round of electrolysis.

CONCLUSION

[0087] Although the foregoing concepts have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatuses. Accordingly, the present examples are to be considered illustrative and not restrictive.