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

This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application 63/631,527 (Attorney Docket No. GRUEP028P) by Xiahui Yao et al., titled: “Methods for Producing Silicon-Containing Structures Using Electrochemically-Generated Solutions and Chemical Reduction”, filed on 2024 Apr. 9, which is incorporated herein by reference in its entirety for all purposes.

This patent application relates generally to methods for producing silicon-containing structures, and more specifically to methods for producing silicon-containing structures using electrochemically generated solutions.

Silicon, germanium, and other materials are important for many applications, such as electronic devices, solar panels, lithium-ion batteries, and grinding medium. However, the production of these materials has been limited to high-temperature processes, such as an ingot grown from a melt and chemical vapor deposition (CVD). These high-temperature processes require significant energy and complex equipment while limiting the type of structures produced using these processes. For example, porosity and other like properties of deposited structures are difficult to control using high-temperature processes.

Low-temperature deposition processes have not been generally available, especially for forming silicon structures used as negative active materials in lithium-ion batteries. At the same time, low-temperature deposition can not only help to reduce production costs but also to fabricate silicon structures with desired characteristics, such as crystallinity, grain sizes, and composition.

What is needed are new methods of forming active materials for electrochemical cells using low temperatures such as electrochemically-generated solutions and chemical reduction of such solutions to generate particles.

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.

Clause 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.

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

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

Clause 4. The method of clause 1, wherein the charge-carrying ions is one or more cations selected from the group consisting of H, Li, Na, K, Cs, tetramethylammonium cation (TMA), tetraethylammonium cation (TEA), tetrapentylammonium cation (TPA), tetrabutylammonium cation (TBA), and 1-butyl-1-methylpyrrolidinium cation (PYR14).

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

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

Clause 7. The method of clause 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.

Clause 8. The method of clause 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.

Clause 9. The method of clause 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.

Clause 10. The method of clause 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.

Clause 11. The method of clause 10, wherein the salt-dissolving solvent comprises a salt-dissolution promoter that is a crown ether.

Clause 12. The method of clause 10, wherein the salt-dissolving solvent comprises a salt-dissolution promoter that is a chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), citric acid, ethylenediamine, and porphine.

Clause 13. The method of clause 10, wherein the salt-dissolving solvent comprises a salt-dissolution promoter that is a chloride selected from the group consisting of boron trichloride (BCl), aluminum chloride (AlCl), gallium chloride (GaCl), lanthanum chloride (LaCl), titanium chloride (TiCl), silicon tetrachloride (SiCl), phosphorus trichloride (PCl), and phosphorus pentachloride (PCl).

Clause 14. The method of clause 10, wherein the salt-dissolving solvent comprises a salt-dissolution promoter that is a halide selected from the group consisting of a fluoride, a bromide, and an iodide.

Clause 15. The method of clause 10, wherein: the cathode solution solvent is diglyme, the cathode solution salt is a sodium perchlorate (NaClO), the redox mediator is naphthalene (CH), the separator is NASCION-type solid electrolyte NaZrSiPO(NZSP), the charge-carrying ions are sodium ions (Na), 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), and the salt-dissolving solvent is water.

Clause 16. The method of clause 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.

Clause 17. The method of clause 1, wherein: the cathode solution solvent is tetrahydrofuran (THF), the cathode solution salt is a lithium hexafluorophosphate (LiPF), the redox mediator is biphenyl (CH), the voltage is −3V, and the silicon-containing precursor is silicon tetrachloride (SiCl).

Clause 18. The method of clause 1, wherein separating the silicon-containing structures from the precursor mixture is performed using a centrifuge or filtration.

Clause 19. The method of clause 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.

Clause 20. The method of clause 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 (PYRTFSI), N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYRTFSI), 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI), 1-butyl-3-methylimidazolium tetrachloroaluminate ([BMIM]AlCl), and acetonitrile (CHN).

Clause 21. The method of clause 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), aluminum chloride (AlCl), tetramethylammonium chloride (TMACl), tetraethylammonium chloride (TEACl), tetrapentylammonium chloride (TPACl), tetrabutylammonium chloride (TBACl), tetrabutylammonium bromide (TBABr), N-butyl-N-methylpyrrolidinium chloride (PYRCl), N-methyl-N-propylpyrrolidinium chloride (PYRCl), lithium hexafluorophosphate (LiPF), sodium hexafluorophosphate (NaPF), lithium perchlorate (LiClO), sodium perchlorate (NaClO), lithium bis(trifluoromethane)sulfonimide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), sodium bis(trifluoromethane)sulfonimide (NaTFSI), and NaFSI.

Clause 22. The method of clause 1, wherein the redox mediator is selected from the group consisting of biphenyl (CH), naphthalene (CH), methylamine (CHNH), a crown ether, metallocene, cobaltocene (Co(ηCH)]), decamethylcobaltocene (CHCo), ferrocene, decamethylferrocene, chromocene, nickelocene, metal carbonyl, nickel tetracarbonyl, iron pentacarbonyl, chromium hexacarbonyl, and dimanganese decacarbonyl.

Clause 23. The method of clause 1, wherein the redox mediator is selected from the group consisting of trivalent neodymium cation (Nd), tetravalent thorium cation (Th), trivalent praseodymium cation (Pr), trivalent erbium cation (Er), trivalent promethium cation (Pm), and trivalent dysprosium cation (Dy).

Clause 24. The method of clause 1, wherein the redox mediator is a metal cation that, when reduced, forms a solid solution selected from the group consisting of LiC, Li—Si alloy, Mg—Si alloy, and Pd—H alloy.

Clause 25. The method of clause 1, wherein the redox mediator is a metal cation that, when reduced, forms a solid material selected from the group consisting of lithium metal, sodium metal, magnesium metal, potassium metal, rubidium metal, cesium metal, calcium metal, strontium metal, lanthanum metal, yttrium metal, praseodymium metal, and cerium metal.

Clause 26. The method of clause 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.

Clause 27. The method of clause 1, wherein the separator is one of: an ion-selective membrane selected from the group consisting of NASCION-structured NaZrSiPO, lithium aluminum titanium phosphate (LATP), Garnet-type lithium lanthanum zirconium oxide (LLZO), and ABO3-type Lithium niobate (LiNbO), 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.

These and other embodiments are described further below with reference to the figures.

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.

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.

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.

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).

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.

is a process flowchart corresponding to methodfor forming silicon-containing structures. Methodmay also be referred to as a low-temperature deposition method and/or liquid-based deposition method as the formation of silicon-containing structuresis performed at low temperatures (e.g., less than 200° C., less than 100° C.) and in liquid phases.

In some examples, methodcomprises (block) providing a cathode solutionand an anode solutionin an reactor, e.g., as schematically shown in. The reactorcomprises a cathode, an anode, and a separator. Various examples of the reactorare within the scope. In some examples, the cathodecomprises a metal, a carbon, a conductive polymer, a conductive ceramic, or a conductive silicon that are stable toward the reduction potential applied here. The anodemay 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 cathodeand the anodecan be connected to the power supply to apply a desired voltage between the cathodeand the anode.

In some examples, the separatorof the reactorcomprises 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 separatormay be one of (a) a dense solid electrolyte selected from the group consisting of NASCION-structured NaZrSiPO(NZSP), lithium aluminum titanium phosphate (LATP), Garnet-type lithium lanthanum zirconium oxide (LLZO), ABO3-type lithium niobate (LiNbO), (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 separatormay 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 separatoris specially selected to allow the charge-carrying ionsthrough the separatorwithout allowing other components through.

The cathode solutioncomprises a cathode solution solvent, a cathode solution salt, and a redox mediator. In some examples, the cathode solution solventis 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 (CHN). 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.

In some examples, the cathode solution saltis selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl), aluminum chloride (AlCl), 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), sodium hexafluorophosphate (NaPF), lithium perchlorate (LiClO), sodium perchlorate (NaClO), 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 saltin the cathode solutionis between 0.05 mol/L and 2 mol/L or, more specifically, between 0.1 mol/L and 1 mol/L.

In some examples, the redox mediatoris selected from the group consisting of biphenyl (CH), naphthalene (CH), methylamine (CHNH), a crown ether (such as 8-crown-4,18-crown-6, dibenzo-18-crown-6), metallocene ((CH)M) such as cobaltocene (Co(ηCH)]), decamethylcobaltocene (CHCo), 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 mediatorin the cathode solutionis 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.

In some examples, the redox mediatoris 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) can be reduced to bivalent neodymium cation (Nd) 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), Th↔Th; trivalent praseodymium cation (Pr), Pr↔Pr; trivalent erbium cation (Er), Er↔Er; trivalent promethium cation (Pm), Pm↔Pm; and trivalent dysprosium cation (Dy), Dy↔Dy. The metal cation may be selected based on, for example, solubility of a salt comprising the cation in the cathode solution solvent, or compatibility of the cation with the cathode solution solvent.

In some examples, the redox mediatoris 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 LiCat−2.84V vs SHE, which is sufficient to reduce silicon halides as a solid solution reducing agent. Other examples may include Li—Si alloy, Mg—Si alloy, Pd—H alloy, etc. The metal cation may be selected based on, for example, solubility of a salt comprising the cation in the cathode solution solvent, the reduction potential of the cation, or compatibility of the cation with the cathode solution solvent.

In some examples, the redox mediatoris 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, the reduction potential of the cation, or compatibility of the cation with the cathode solution solvent.

The anode solutioncomprises an anode solution solventand an anode solution salt. In some examples, the anode solution solventis 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 (CHN), an amide, an ester, a sulfone, a sulfoxide, and water. In some examples, the anode solution saltis selected from the group consisting of lithium chloride (LiCl), sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl), aluminum chloride (AlCl), 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), sodium hexafluorophosphate (NaPF), lithium perchlorate (LiClO), sodium perchlorate (NaClO), 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 saltin the anode solutionis between 0.05 mol/L and 2 mol/L or, more specifically, is between 0.1 mol/L and 1 mol/L.

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