Patent Application
20220190324
This application relates to nanoparticles having improved electroactive cores encapsulated in shells, such as those that may be used as electrode materials for secondary batteries or other energy storage devices, and methods of making same.
A major objective in commercial development of next generation rechargeable batteries is to provide batteries with higher energy densities and lower cost than state of the art lithium ion batteries. One of the most promising approaches towards achieving this goal relies on use of a sulfur cathode. Sulfur is extremely attractive because it is inexpensive, abundant, and offers a theoretical charge capacity that is an order of magnitude higher than conventional metal oxide-based intercalation cathodes used in current lithium ion cells. However, manufacture of a practical sulfur battery has been an elusive goal. Among the numerous challenges that plague sulfur cathodes, some of the most serious arise from: (1) the fact that sulfur is insoluble and electrically insulating; and (2) that lithium polysulfide intermediates, which form during battery discharge are highly soluble in electrolyte and difficult to retain at a cathode. The first challenge presented leads to high impedance and low sulfur utilization, while the second leads to a polysulfide shuttle that decreases battery efficiency and leads to anode fouling.
Thus, although elemental sulfur has been under investigation as a battery cathode material for more than 50 years, certain fundamental challenges have yet to be overcome. To produce a viable cathode material, conductivity of elemental sulfur must first be enhanced. Unlike commercial lithium ion cathodes containing LiCoO2, which possess a high electronic conductivity and do not require significant addition of conductive additives, sulfur is an insulator, and therefore, in order to prepare a viable and commercially useful battery based on an elemental sulfur cathode, active material must be present in a structure that enhance electrochemical accessibility of sulfur. Attempts to address these challenges have included use of nanoporous monoliths and engineered nanomaterials such as core shell particles, nanotubes and laminates.
In addition, diffusion and subsequent loss of polysulfide intermediates that are formed during electrochemical cycling must be controlled to produce a viable electrochemical cell. During discharge, sulfur reduces in a stepwise manner by forming a series of polysulfide intermediates that are ionic in nature, dissolve readily in the electrolyte, and may be lost by migration to the anode causing mass loss of active material upon cycling and anode fouling.
To date, technical approaches taken to address and solve these fundamental challenges have resulted in diminished charge capacity in comparison to the theoretical value of sulfur.
Although incremental improvements in capacity and cycle life have been made, significantly greater improvements in stabilization of elemental sulfur to polysulfide loss is necessary in order to produce commercially viable metal-sulfur batteries. Thus, there is a need for a sulfur active material which allows for complete utilization of sulfur while minimizing loss of polysulfide.
An objective of the present disclosure is to provide structured nanomaterials suitable for utilization as a cathode active material that is capable of providing good sulfur utilization, while effectively containing soluble polysulfides to prevent their loss or migration.
Generally, this disclosure relates to improved nanoparticles, such as those for use in electrodes for an energy storage device. In particular, the disclosure relates to improved nanoparticle cores and methods of making same. The present invention provides, among other things, nanoparticles comprising a shell (e.g., polymeric, inorganics, elemental carbon, metal oxides, sulfides, etc.) and an electroactive core comprising polythionate molecules (e.g. compounds with a structure −O3S—(S)n—SO3−) disposed within the shell. In some embodiments, an electroactive core comprises compounds with a structure (S)n—SO3−. The present invention encompasses the recognition that such nanoparticles offer particular advantages as cathode materials. For example, nanoparticle cores that comprise polythionates and/or other non-electrochemically active compounds can improve battery performance (e.g., battery capacity may vary depending on the Sn to SO3− ratio in the particles). These cores, in some embodiments, comprise monodispersed constituents with particle size and solubility optimized for particular applications. Additionally, in some embodiments,—SO3− anions are lithiated and aid in ion conductivity, solvation, and polysulfide trapping. Furthermore, in some embodiments, anionic polythionate particle surfaces may template polymer shell synthesis and/or dope a polymer (e.g., polyaniline). Further advantages of nanoparticles having such cores are explained below.
In one aspect, the present disclosure relates to a core-shell nanoparticle including a shell defining an internal volume and a sulfur-based core with a composition different from said shell and disposed within an internal volume defined by said shell. In some embodiments, a sulfur-based core includes polythionate and/or another electrochemically active compound, along with optional additives.
In another aspect, the present disclosure relates to a core-shell nanoparticle including a shell defining an internal volume and a core made up of a composite of lithium thiosulfate and lithium sulfide disposed within an internal volume defined by said shell.
In yet another aspect, the present disclosure relates to an electrode for an energy storage device, such as a secondary battery, a capacitor, or other electrochemical system. In some embodiments, an electrode includes a nanoparticle as described herein, for example, having a core-shell nanoparticle including a shell defining an internal volume and a sulfur-based core with a composition different from said shell and disposed within an internal volume defined by said shell. In some embodiments, the sulfur-based core includes polythionate.
In another aspect, the present disclosure relates to an energy storage device including an anode, a cathode having a core shell nanoparticle with a shell defining an internal volume and a sulfur-based core including polythionate disposed within an internal volume defined by said shell, a separator, and an electrolyte. In still another aspect, the present disclosure relates to an energy storage device including an anode, a cathode including lithium polythionate, a separator, and an electrolyte. It will be appreciated that when charged, such an energy storage device will contain a cathode substantially free of lithium (e.g., comprising an anode, a cathode including lithium polythionate, a separator, and an electrolyte).
In various embodiments of the foregoing aspects, a sulfur-based core includes a polythionate composition having a plurality of molecules of formula −O3S—(S)n—SO3−, wherein n is on average in the composition between about 1 and about 40 (e.g., between about 4 and about 40). In various embodiments of the foregoing aspects, a sulfur-based core includes a composition having a plurality of molecules of formula (S)n—SO3−, wherein n is on average in said composition between about 1 and about 40. In some embodiments, a sulfur-based core includes elemental sulfur. In some embodiments, a mass ratio of polythionate to elemental sulfur in a sulfur-based core is between about 1:10 and about 10:1. In various embodiments, a molar ratio of —SO3− functional groups to S0 atoms in the core is at least about 1:200, preferably, at least about 1:100, at least about 1:50, at least about 1:40, or at least about 1:25).
In some embodiments, a core-shell nanoparticle includes lithium (Li), such as lithium polythionate; however, counter ions other than Li, such as sodium, potassium, rubidium, magnesium, zinc or calcium are contemplated and considered within the scope of the present disclosure. In some embodiments, a core can also include one or more conductive additives, such as carbon, graphite, carbon nanotubes, graphene, metal oxides, metal chalcogenides, and conductive polymers. In some embodiments, a sulfur-based core includes a composite of elemental sulfur, polythionate and a conductive additive. In certain embodiments of core-shell nanoparticles described herein, polythionate comprises less than about 25 wt. % of a core, less than about 20 wt. %, less than about 15 wt. %, or less than about 10 wt. %. In some embodiments, polythionate comprises about 1 to about 10 wt. % of a core. Generally, it is desirable for about 80% or greater of the nanoparticle to comprise electroactive material.
In some embodiments, a core-shell nanoparticle includes a polymer shell, which in some instances comprises a conductive polymer. In some embodiments, a polymer shell comprises polyaniline. Additionally or alternatively, a shell can include an inorganic material. In various embodiments, a shell can include a transition metal oxide, such as manganese dioxide (MnO2), iron oxide black or magnetite (Fe3O4), titanium dioxide (TiO2), or molybdenum trioxide (MoO3), In some embodiments, a shell can include a transition metal sulfide, such as titanium disulfide (TiS2), molybdenum disulfide (MoS2), iron sulfide (FeS), greigite (Fe3S4), or iron disulfide (FeS2). In various embodiments, a shell includes a composite of one or more polymers with at least one conductive additive, such as carbon, graphite, carbon nanotubes, graphene, metal oxides, metal chalcogenides, or metal-organic frameworks.
In some embodiments, a dimension (e.g., a diameter or length) of a nanoparticle is between about 20 and about 1500 nm, between about 20 and about 1000 nm, between about 200 and about 1200 nm, between about 100 and about 900 nm, between about 200 and about 800 nm, or between about 400 and about 800 nm. In some embodiments, the nanoparticle is substantially spherical, a nanowire, or a plate; however, other shapes (e.g., ovoid, polyhedral, of irregular shape, or combination thereof) are contemplated and considered within the scope of the present disclosure. In various embodiments, a sulfur-based core occupies only a portion of an internal volume defined by a shell, for example, a sulfur-based core occupies between about 20% and 80% of an internal volume defined by a shell. In some embodiments, a shell of a nanoparticle has a thickness of between about 5 and about 50 nm, and in some instances, includes two or more layers. In some embodiments, two or more layers of a nanoparticle shell have different compositions, for example, in some such embodiments, at least one layer is a polymer and at least one layer is an elemental carbon or an inorganic composition.
In another aspect, the present disclosure relates to a method of producing a sulfur-based composite comprising polythionate. For example, a method can include compositing elemental sulfur with lithium thiosulfate or partially oxidizing an elemental sulfur nanoparticle. These composites can be provided in various forms, such as powders, pellets, or in a solution as a slurry, with or without other additives.
In some embodiments, a method is directed to producing a core-shell nanoparticle. In some embodiments, a method includes steps of: introducing sodium thiosulfate to an acid solution (e.g., hydrochloric, formic, or sulfuric); reacting sodium thiosulfate in solution to precipitate sulfur-based core materials (e.g., a finished core or an intermediate product); and controlling said reaction. In some embodiments, a reaction can be controlled by at least one of: introducing an oxidizing agent to said acid solution, adjusting a pH of said solution, varying an environmental condition (e.g., solution and/or ambient temperature, exposure to UV radiation, etc.), adding a surfactant to said acid solution, limiting mixing time (e.g., continuous vs. intermittent stirring, stirring during a gradual introduction of sodium thiosulfate); or washing (e.g., rinsing core material or introducing core material to another solution). In some embodiments, potassium thiosulfate may be used instead of sodium thiosulfate.
In various embodiments, a method further includes a step of introducing a polymer to the acid solution to encapsulate a core (e.g., 1% (weight ratio) of polyvinylpyrrolidone (PVP), Mw about 40,000). In certain embodiments, precipitated cores can undergo further processing, such as introduction to additional solutions comprising additional polymers (e.g., polyaniline) or other additives (e.g., a conductive carbon), to finalize the core. In some embodiments, an oxidizing agent is selected from the group consisting of a peroxymonosulfate [HSO5]−, a permanganate [MnO4]−, a dichromate [Cr2O7]2−, a chromate [CrO4]−, a bromate salt [BrO3]−, a hypochlorite salt [ClO]−, a chlorate [ClO3]−, a perchlorate [ClO4]−, a periodate [IO4]−, a vanadate salt, or a nitrate [NO3]−, where a counter cation is selected from the group consisting of Li+, Na+, K+, Ag+, Mg2+, or Ca2+, a metal oxide, such as MnO2, CrO3, OsO4, RuO4, or oxygen. In a particular embodiment, a step of controlling a reaction includes steps of maintaining pH of reaction solution below 7 (e.g., by adding additional acid during the reaction) and bubbling an oxidizing agent (e.g., oxygen) through said solution.
In another aspect, the present disclosure relates to a powder for use in making an electrode, where a powder includes a mixture of nanoparticles as disclosed herein and electrically conductive particles. In some embodiments, a mixture further comprises a binder and/or a mixture is homogenous.
These and other objects, along with advantages and features of disclosed systems and methods, will become apparent through reference to the following description and accompanying drawings. Furthermore, it is to be understood that features of various embodiments described are not mutually exclusive and can exist in various combinations and permutations.
In order for the present disclosure to be more readily understood, certain terms are first defined below. Additional definitions for the following terms and other terms are set forth throughout the specification.
In this application, unless otherwise clear from context, the term “a” may be understood to mean “at least one.” As used in this application, the term “or” may be understood to mean “and/or.” In this application, the terms “comprising” and “including” may be understood to encompass itemized components or steps whether presented by themselves or together with one or more additional components or steps. As used in this application, the term “comprise” and variations of the term, such as “comprising” and “comprises,” are not intended to exclude other additives, components, integers or steps.
About, Approximately: As used herein, the terms “about” and “approximately” are used as equivalents. Unless otherwise stated, the terms “about” and “approximately” may be understood to permit standard variation as would be understood by those of ordinary skill in the art. Where ranges are provided herein, endpoints are included. Any numerals used in this application with or without about/approximately are meant to cover any normal fluctuations appreciated by one of ordinary skill in the relevant art. In some embodiments, the term “approximately” or “about” refers to a range of values that fall within 25%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, or less in either direction (greater than or less than) of the stated reference value unless otherwise stated or otherwise evident from the context (except where such number would exceed 100% of a possible value).
Electroactive Substance: As used herein, the term “electroactive substance” refers to a substance that changes its oxidation state, forms alloys, or partakes in a formation or breaking of chemical bonds, in a charge-transfer step of an electrochemical reaction.
Polymer: As used herein, the term “polymer” generally refers to a substance that has a molecular structure consisting chiefly or entirely of repeated sub-units bonded together, such as synthetic organic materials used as plastics and resins.
Nanostructure, Nanomaterial: As used herein, these terms may be used interchangeably to denote a composition with sub-micrometer features. Such materials can have essentially any shape or configuration, such as a tube, a wire, a laminate, sheets, lattices, a box, a core and shell, or combinations thereof.
Nanoparticle: As used herein, refers to a discrete particle with at least one dimension having sub-micron dimensions.
Substantially: As used herein, the term “substantially” refers to the qualitative condition of exhibiting total or near-total extent or degree of a characteristic or property of interest.
In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating principles of disclosed systems and methods and are not intended as limiting. For purposes of clarity, not every component may be labeled in every drawing. In the following description, various embodiments are described with reference to the following drawings, in which:
Generally, the present disclosure is directed to compositions (e.g., nanoparticles, powder mixtures, slurries) for use in energy storage devices and related methods for fabricating such compositions. In certain embodiments, a provided composition comprises a nanoparticle in the form of a core-shell structure with an electroactive core surrounded by a shell. In certain embodiments, a void space is interposed between core and shell. Without wishing to be bound by any particular theory, it is believed that void space reduces or eliminates mechanical stresses associated with volumetric expansion of an electroactive core during cycling of a battery. Methods described herein include forming an electroactive core and modifying the same. Provided nanoparticles are useful within energy storage devices, such as secondary batteries (e.g., a lithium-sulfur battery), a capacitor, or other electrochemical system.
I. Nanostructured Materials
Nanostructured materials of the present disclosure are not limited to any specific morphology. Provided nanostructured materials may take various forms; a few non-limiting examples of which are illustrated in
In the discussion below, while most explanations and examples pertain to core-shell nanoparticles, these are presented as non-limiting examples of a specific morphology in which provided polythionate compositions can be utilized to fabricate electrodes and electrochemical cells. Various alternative examples of provided compositions can include bulk powder mixtures, such as a ball-milled mixture of sulfur, Li2S2O3, and carbon black, or slurries (e.g., various powder mixtures combined with one or more solvents) that can be used to produce electrodes.
In some embodiments, provided composites include a sulfur-based core (e.g., a core comprising lithium thiosulfate and sulfur or lithium sulfide) and a shell. Without wishing to be bound by any particular theory, it is believed that, in some embodiments, a provided sulfur-based core reacts electrochemically with metal ions during battery operation, e.g., to accept the metal ions to form a metal-sulfide during discharge of a battery and to release metal ions from a metal-sulfide during charging of the battery. In some embodiments, a shell at least partially encases a sulfur-based core and is formed from a material, such as a polymer, an inorganic material, elemental carbon, metal oxides, sulfides, or combinations thereof. As used herein, the term “polythionate” refers to a compound of formula SxO62−, where x is ≥3. The term polythionate as used herein may also refer to compounds of formula SxO3− (e.g. (S)nSO3−). In certain embodiments, compositions of the present disclosure comprise equilibrium mixtures of SxO62− and SxO3−.
In certain embodiments, provided nanostructured materials are characterized in that an electroactive substance is in a form having nanometer dimensions. In certain embodiments, an electroactive substance is present in a form having at least one dimension with a length in a range of about 5 to about 1,000 nm. In certain embodiments, an electroactive substance is present in a form having at least one dimension with a length in a range of about 10 to about 50 nm, about 30 to about 100 nm, about 100 to about 500 nm, or about 500 to about 1,000 nm. In certain embodiments, an electroactive substance is present in a form having at least one dimension with a length in a range of about 5 to about 1,000 nm.
Typically, electroactive cores for lithium-sulfur batteries are made of a sulfur material, such as elemental sulfur, S8, metal sulfides, sulfur-containing polymers, or organic molecules. Shells of conventional nanoparticle shells are made of an organic polymer material, such as polyaniline or other organic polymers capable of providing a shell to facilitate lithium ion transport and sulfur material vapor transport as required during operation; however, other shell materials as previously disclosed are contemplated and considered within the scope of the present disclosure.
Generally, dimensions and shape of provided compositions (e.g., a nanoparticle) will vary to suit a particular application. In some embodiments, a nanoparticle has a dimension (e.g., diameter or length) in a range of about 20 nm to about 1,000 nm, with a wall thickness in a range of about 5 nm to about 50 nm. In certain embodiments, core size and shape is varied to suit a particular application and, in some instances, has a dimension in a range of about 200 nm to about 300 nm. Generally, a core will take up about 20% to about 80% of a volume of an internal cavity 18, depending on charge/discharge status of an electrode or energy storage device containing provided nanoparticles.
Alternative nanoparticles, or nanostructures, 100, 200, 300 are depicted in
As previously discussed, a nanoparticle has a thin shell 514 that retains metal sulfide, polysulfide, and polythionate materials within nanoparticle core 512A. Thus, shell 514 prevents migration of polysulfides (and therefore sulfur) out of a core-shell nanoparticle. In some embodiments, a shell 514 has a thickness of approximately 1-10 nm and makes up about 5-10% of a core-shell nanoparticle by weight. Typically, a shell 514 will be electrically and/or ionically conductive to enable electrons and/or ions to pass into and out of a nanoparticle core 512A.
When a core-shell nanoparticle 510 is used in a Li/S cell, provided Li/S cell may be charged 565, which causes lithium atoms to be extracted from its core-shell nanoparticle and migrated to a negative electrode. While a Li/S cell is charged, its core-shell nanoparticle has a charged state (510B). In a charged state, a core of a core-shell nanoparticle is a sulfur-based nanoparticle core 512B. In some embodiments, a sulfur nanoparticle core 512B occupies a volume that is smaller than a volume occupied by lithium sulfide-based nanoparticle core 512A. In some embodiments, shell 514 has sufficient structural strength or elasticity to accommodate a volume change that occurs during a charge/discharge process.
A Li/S cell may be discharged 570, which causes lithium atoms to migrate back into core-shell nanoparticle 510A. This causes a core of a core-shell nanoparticle to return to a lithium sulfide-based nanoparticle core 512A. In a transition from a sulfur-based nanoparticle core 512B to a lithium sulfide-based nanoparticle core 512A, a core grows back to approximately its original size and composition.
Inclusion of polythionates in an improved core allows a nanoparticle to be “tuned” to impact performance. A challenge in developing sulfur cathodes is that their capacity fades over multiple cycles. Without being bound to any particular theory, it is believed that observed loss of capacity is at least partially due to sulfur becoming physically and electrochemically inaccessible during cycling. Given that sulfur batteries are conversion cathodes, (e.g., cathodes whose operation requires repeated consumption and reforming of new chemical compounds), and the fact that end-members of multistep redox reaction of sulfur (S8 and Li2S) are both insoluble and highly electrically insulating, it is critical that deposition of these compounds be controlled to avoid depositing sulfur or Li2S in forms or locations where it will not be in good contact with electrical or lithium ion conducting substances. Such materials become stranded and are lost to further conversion causing capacity fade. Without being bound by theory, or thereby limiting the scope of the present disclosure, it is believed that presence of thionate groups in a sulfur cathode prevents these undesirable phenomena by modulating dynamics of sulfur consumption and re-deposition in the cathode, thereby providing sulfur cathodes that suffer less capacity fade over time. This effect varies as ratio of thionate functional groups to elemental sulfur atoms in a particles varies. In some embodiments, a mass ratio of —SO3− functional groups to S0 atoms (e.g., atoms in Sn) in a core is at least about 1:200 and may be as high as about 1:5. Since content of elemental sulfur atoms (S0) will vary as an energy storage device cycles through discharge and charge cycles and since an electroactive elemental sulfur may be partially or wholly reduced to S2− during cycling, in certain embodiments herein a thionate to sulfur ratios described refer to a ratio of —SO3− groups to a sum of S0 and S2− atoms present. For example, as a device discharges and cathodic sulfur reduces to Li2S or other reduced species. During a charge cycle, a core reforms returning to substantially the same ratio of —SO3− functional groups to S0 atoms, though specific chemical atoms may be different before and after a cycle.
Nanostructured materials of the present disclosure comprise an electroactive substance. An electroactive substance is preferably in a form having nanometer dimensions. In certain embodiments, an electroactive substance is present in a form having at least one dimension with a length in a range of about 5 to about 1,000 nm. In certain embodiments, the electroactive substance is present in a form having at least one dimension with a length in a range of about 10 to about 50 nm, about 30 to about 100 nm, about 100 to about 500 nm, or about 500 to about 1,000 nm. In certain embodiments, an electroactive substance is present in a form having at least one dimension with a length in a range of about 400 to about 1,000 nm.
In certain embodiments, provided nanostructured materials have utility as cathode materials for sulfur batteries. Such compositions necessarily comprise an electroactive sulfur-based material. Examples of suitable electroactive sulfur materials include elemental sulfur, sulfur-containing organic molecules, polymers or composites, or metal sulfides as well as combinations or composites of two or more of these.
In certain embodiments, electroactive sulfur is present in the form of elemental sulfur. In certain embodiments, electroactive sulfur material comprises S8.
In certain embodiments, electroactive sulfur is present as a metal sulfide. In certain embodiments, a metal sulfide comprises an alkali metal sulfide; in certain embodiments, a metal sulfide comprises lithium sulfide.
In certain embodiments, electroactive sulfur material is present as a composite with another material. Such composites may include conductive additives such as graphite, graphene, metal sulfides or oxides, or conductive polymers. In certain embodiments, sulfur may be alloyed with other chalcogenides such as selenium or arsenic.
Generally, dimensions and shape of electroactive sulfur-based material in a provided cathode composition may be varied to suit a particular application and/or be controlled as a result of nanostructure morphology. In various embodiments, electroactive sulfur-based material is present as a nanoparticle. In certain embodiments, such electroactive sulfur-based nanoparticles have a spherical or spheroid shape. In certain embodiments, nanostructured materials of the present disclosure comprise substantially spherical sulfur-containing particles with a diameter in a range of about 50 to about 1,200 nm. In certain embodiments, such particles have a diameter in a range of about 50 to about 250 nm, about 100 to about 500 nm, about 200 to about 600 nm, about 400 to about 800 nm or about 500 to about 1,000 nm.
Such nanoparticles may have various morphologies as described above. In certain embodiments, electroactive sulfur-based material is present as a core of a core-shell particle, where it is surrounded by a conductive shell. In certain embodiments, such core-shell particles may comprise yolk-shell particles as described above.
II. Cathode Mixtures
Generally, various nanostructures disclosed herein can be used to produce cathodes. Cathode production typically involves applying a uniform layer of a cathode mixture onto a current conductor such a metal foil or conductive carbon sheet. In certain embodiments, the present disclosure provides cathode mixtures that are useful for producing and manufacturing cathodes for batteries or other electrochemical devices. Provided cathode mixtures include nanostructured materials according to embodiments and examples herein (e.g., nanowires, core-shell particles, etc.) optionally mixed with additional materials such as electrically conductive additives, binders, surfactants, stabilizers, wetting agents and the like. Such mixtures are typically provided in the form of fine powders that can be applied by techniques such as slurry coating or roll-to-roll processing. Cathode mixtures typically comprise relatively larger quantities of materials than materials made for experimental evaluation and it can be non-trivial to produce nanostructured materials with consistent characteristics in large batches. In certain embodiments, cathode mixtures of the present disclosure are characterized in that they comprise a homogenous sample with a quantity greater than about 100 grams (g), greater than about 1 kilogram (kg), greater than about 10 kg, greater than about 100 kg, or greater than about 1 ton.
In various embodiments, additional materials may be included with nanostructured materials to alter or otherwise enhance provided cathode mixtures produced from a mixture. Generally, provided cathode mixtures will contain nanoparticles in a proportion ranging from about 50 wt. % to about 98 wt. %, preferably about 60 wt. % to about 95 wt. %, and more preferably about 75 wt. % to about 95 wt. % of a total cathode mixture.
In certain embodiments, cathode mixtures comprising provided nanoparticles contain at least 50 wt. % sulfur relative to all components in a cathode mixture. In certain embodiments, provided cathode mixtures are characterized in that they have a high sulfur content. In certain embodiments, provided cathode mixtures are characterized in that they contain above about 75 wt. %, above about 80 wt. %, above about 85 wt. %, or above about 90 wt. % sulfur relative to total cathode mixture.
In certain embodiments, provided nanostructured materials are mixed with electrically conductive particles (e.g., conductive carbon, such as carbon black, graphene, etc.) and a binder. Typical binders include polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE), polyacrylates, polyvinyl pyrrolidone, (PVP) poly(methyl methacrylate) (PMMA), copolymers of polyhexafluoropropylene and polyvinylidene fluoride, polyethyl acrylate, polyvinyl chloride (PVC), polyacrylonitrile (PAN), polycaprolactam, polyethylene terephthalate (PET), polybutadiene, polyisoprene or polyacrylic acid, or derivatives blends or copolymers thereof. In some embodiments, a binder is water soluble binder, such as sodium alginate, or carageenan. Generally, binders hold active materials together and in intimate contact with a current collector (e.g., aluminum foil or copper foil, carbon paper or fabric).
In certain embodiments, cathode powder mixtures can be provided without a binder, which can be added during a manufacturing process to produce electrodes (e.g., as a solution or dispersion in water or a suitable carrier).
In various embodiments, a cathode mixture is ground, powdered or mixed to control properties of a cathode powder mixture (e.g. particle size) and to thoroughly mix ingredients. Such mixing can be performed by any means known in the art including but not limited to pin milling, hammer milling, jet milling, ball milling, air classifying, and combinations of these. Specific means used for mixing a cathode powder mixture will vary to suit a particular application, such as large scale production of cathode mixtures (e.g., production of drum quantities of powder for sale to cathode manufacturers).
Various materials for use in cathode mixtures are disclosed in Cathode Materials for Lithium Sulfur Batteries: Design, Synthesis, and Electrochemical Performance, Lianfeng, et al., Interchopen.com, Published June 1st 2016 and The strategies of advanced cathode composites for lithium-sulfur batteries, Zhou et al., SCIENCE CHINA Technological Sciences, Volume 60, Issue 2: 175-185(2017), the entire disclosures of which are hereby incorporated by reference herein.
III. Electrode Compositions
There are a variety of methods for manufacturing electrodes for use in a LiS battery. One such process, referred to as a “wet process,” involves adding a positive active material (i.e., the nanostructured materials), a binder and a conducting material (i.e., the cathode mixture) to a liquid to prepare a slurry. Compositions are typically formulated into a viscous slurry in order to facilitate a downstream coating operation. A thorough mixing of a slurry can be critical for coating and drying operations, which will eventually effect performance and quality of electrodes. Appropriate slurry mixing devices include ball mills, magnetic stirrers, sonication, planetary mixers, high speed mixers, homogenizers, universal type mixers, and static mixers. In certain embodiments, a liquid is any liquid that effectively disperses positive active material, binder, conducting material, and any additives homogeneously, and is easily evaporated. Possible slurrying liquids include, for example, N-methylpyrrolidone, acetonitrile, methanol, ethanol, propanol, butanol, tetrahydrofuran, water, isopropyl alcohol, dimethylacetamide and the like.
In certain embodiments, a prepared composition is coated on a current collector and dried to form a positive electrode. Specifically, a slurry is used to coat an electrical conductor to form an electrode by evenly spreading a slurry on to a conductor, which may then optionally be roll-pressed, calendared, and heated as is known in the art. Generally, a dried slurry forms a matrix held together and adhered to a conductor by a polymeric binder included in a cathode mixture. In certain embodiments, a matrix comprises a lithium conducting polymer binder, such as polyvinylidene difluoride (PVDF), styrene butadiene rubber (SBR), polyethylene oxide (PEO), polyacrylic acid, polyacrylates, carageenan, and polytetrafluoroethylene (PTFE). In some embodiments, additional carbon particles, carbon nanofibers, carbon nanotubes, etc. are dispersed in a matrix to improve electrical conductivity. In some embodiments, lithium salts are dispersed in a matrix to improve lithium conductivity.
In certain embodiments, a current collector is any suitable material with good electronic conductivity. In certain embodiments, the current collector is selected from the group consisting of: aluminum foil, copper foil, nickel foil, stainless steel foil, titanium foil, zirconium foil, molybdenum foil, nickel foam, copper foam, conductive carbon papers, sheets, or fabrics, polymer substrates coated with conductive metal, and/or combinations thereof.
In certain embodiments, thickness of a matrix may range from a few microns to hundreds of microns (e.g., 2-200 microns). In some embodiments, a matrix has a thickness of about 10 to about 50 microns. Generally, increasing thickness of a matrix increases percentage of active materials relative to other cell constituents by weight, and may increase cell capacity. However, diminishing returns may be exhibited beyond certain thicknesses. In some embodiments, a matrix has a thickness of between about 5 and about 200 microns. In some embodiments, a matrix has a thickness of between about 10 and about 100 microns.
A negative electrode (i.e., anode) contains a negative active material. In some embodiments, a negative active material is one that can reversibly release lithium ions. In some embodiments, a negative active material may be lithium metal or a lithium composite with other materials such as carbon, tin, titanium, silicon, and mixtures, alloys, or composites of any of these. Suitable carbon materials include crystalline carbon, amorphous carbon, graphitic carbon, graphene, carbon nanotubes, or a combination thereof. Other suitable materials, which can reversibly form a lithium-containing compound by reacting with lithium or its ions, may include tin oxide (SnO2), titanium nitrate, silicon (Si), and the like, but not limited thereto. Lithium metal may be present in pure form or alloyed. Lithium alloys may include lithium and metal selected from the group consisting of: Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Zn, Al and Sn. Typically, a negative electrode may also contain negative active material disposed on a current collector, such as those described above.
PCT Publication Nos. WO2015/003184, WO2014/074150, and WO2013/040067, the entire disclosures of which are hereby incorporated by reference herein, described various methods of fabricating electrodes and electrochemical cells that are suitable to utilize nanostructured materials of the present invention.
IV. Electrochemical Cells
Generally, an electrochemical battery such as an Li/S battery comprises a stack of electrodes comprising a plurality of individual electrochemical cells.
V. Methods
In various embodiments, a core is manufactured in a fully charged state, as opposed to a conventional discharged state. However, a core can be made in a partially charged state as well to suit a particular application.
After particle 10 has been produced with core 12 encapsulated by a polyaniline shell (b), core 12 can be partially removed (e.g. by vacuum or dissolution) to separate core 12 from shell 14 and define a void space 18 therein. In some embodiments, an interim coated nanoparticle 10 as shown in (c) is obtained through oxidation with ammonium persulphate at 0° C. for 24 h. As previously described, in some embodiments, a shell is formed by materials other than polymers.
In alternative embodiments, sulfur-based cores can be produced in a bulk state and may be mechanically milled (e.g., ball milled) to reduce particle sizes down to nanoscale particles. Such mechanical milling should be performed under a neutral (inert) gas atmosphere. Other examples of basic methods for forming nanoparticle cores are described in PCT publication number WO2014/074150, the entire disclosure of which is hereby incorporated by reference herein.
In certain embodiments, an electroactive sulfur composition is produced by milling elemental sulfur in the presence of polythionate or thiosulfate salts to provide a composition with a controlled ratio of S0 atoms to —SO3− groups. In certain embodiments, a sulfur based core is produced by mechanically milling elemental sulfur with lithium polythionate. In certain embodiments, a sulfur based core is produced by mechanically milling elemental sulfur with lithium thiosulfate. In certain embodiments a ratio of elemental sulfur to thionate or thiosulfate salt is controlled such that a ratio of S0 atoms to —SO3− groups in a composition is between about 4:1 and about 500:1. In certain embodiments this ratio is between about 10:1 and about 50:1, between about 20:1 and about 100:1, or between about 50:1 and about 200:1.
In certain embodiments, an electroactive sulfur composition is produced by milling lithium sulfide or lithium polysulfides with polythionate or thiosulfate salts to provide a composition with a controlled ratio of S2 atoms to —SO3− groups. In certain embodiments, a sulfur based core is produced by mechanically milling lithium sulfide with lithium polythionate. In certain embodiments, a sulfur based core is produced by mechanically milling lithium sulfide with lithium thiosulfate. In certain embodiments a ratio of lithium sulfide to thionate or thiosulfate salt is controlled such that a ratio of S2 atoms to —SO3− groups in a composition is between about 4:1 and about 500:1. In certain embodiments this ratio is between about 10:1 and about 50:1, between about 20:1 and about 100:1, or between about 50:1 and about 200:1.
Typically, freshly prepared sulfur-based particle(s) is subjected to further processes to encapsulate a core within a shell and produce a finished nanoparticle. In some embodiments, sub-micron sulfur-based particles are generated in-situ from reaction of sodium thiosulfate with hydrochloric acid in the presence of specific polymers that encapsulate formed sulfur particles. Sulfur generating reaction shown in
As discussed above, a sulfur-based particle can be prepared via reaction of a thiosulfate salt (e.g. lithium, sodium, or potassium thiosulfate) with an acid; however, it is also possible to prepare a sulfur-based particle via reaction of SO2 or a sulfite salt (e.g. Na2SO3, K2SO3, CaSO3, Li2SO3, or MgSO3) with hydrogen sulfide in water. Alternatively, a sulfur-based particle can be prepared by pouring a solution of elemental sulfur Sn dissolved in an organic solvent (e.g. carbon disulfide, benzene, toluene, fluorobenzene, etc.) into an aqueous solution to facilitate precipitation. In some embodiments, an aqueous solution contains one or more salts. In some such embodiments, an aqueous solution further contains an oxidizing agent for in situ oxidation of a sulfur-based particle as it forms or a precipitated sulfur-based particle may be subsequently oxidized via reaction with an oxidizing agent to generate —SO3− groups in a sulfur-based particle. In certain embodiments, such oxidizing steps result in formation of polythionates within the particle.
In various embodiments, synthetic parameters are modulated to tune a polythionate to elemental sulfur ratio of a sulfur-based particle to achieve a preferred composition. For example, in some embodiments, an oxidizing agent is present in a reaction mixture.
A selected oxidizing agent can be a salt, such as, a peroxymonosulfate [HSO5]−, a permanganate [MnO4]−, a dichromate [Cr2O7]2−, a chromate [CrO4]−, a bromate salt [BrO3]−, a hypochlorite salt [ClO]−, a chlorate [ClO3]−, a perchlorate [ClO4]−, a periodate [IO4]−, or a nitrate [NO3]−, where the counter cation is selected from the group consisting of Li+, Na+, K+, Ag+, Mg2+, or Ca2+. In some embodiments, an oxidizing agent is a metal oxide, such as MnO2, CrO3, OsO4, RuO4, or Fe2O3. In some embodiments, hydrogen peroxide or an organic peroxide or peroxy-acid is used as an oxidant. In some embodiments, oxygen gas and/or ozone is added as an oxidizing agent (e.g., bubbled through a solution).
In certain embodiments, pH of a reaction is preferably about 7 or less. In some embodiments, pH is modulated via addition of acid to be in a range of about 3 to 7, or about 4 to 6.
In some embodiments, particles are stirred at room temperature. In some embodiments, particles are stirred at low temperature, such as in an ice bath. In some embodiments, heat is applied (e.g., about 30° C., about 40° C., about 50° C., about 60° C., about 70° C., or about 80° C.).
In some embodiments, a reaction mixture is exposed to ultraviolet (UV) radiation. In some embodiments, a reaction mixture is stirred in the dark. In some embodiments, a reaction mixture is stirred in ambient light. Generally, percentage of polythionates present in a finished sulfur-based particle can be varied by varying conditions under which such particle is formed, for example, exposure UV radiation accelerates aging of a particle. In addition to conditions identified above, a reaction can be controlled by adding a surfactant, varying a mixing protocol (e.g., continuous or intermittent stirring), or washing of particles.
In some embodiments, sulfur-based particles are isolated by precipitation. In some embodiments, control of polythionate to elemental sulfur ratio of sulfur-based particles is afforded via precipitation of a composite after stirring for various lengths of time, such as within about 1 to about 3 hour (h), about 2 to about 7 h, about 5 to about 12 h, about 10 to about 20 h, about 16 to about 24 h, about 24 to about 48 h, or about 48 to about 192 h.
In some embodiments, a sulfur-based core is aged post-synthesis by treatment with a solvent that preferentially dissolves sulfur but not polythionate. Suitable solvents for such processes include, but are not limited to cyclohexane, CS2, benzene, toluene, etc. and mixtures containing any of these. In some embodiments, a process can comprise a step of contacting a sulfur-based core with such solvent for a proscribed period; for example about 1 to about 3 h, about 2 to about 7 h, about 5 to about 12 h, about 10 to about 20 h, about 16 to about 24 h, about 24 to about 48 h, or about 48 to about 192 h. In certain embodiments, a solution is stirred at room temperature. In some embodiments, temperature of a contacting step is controlled—for example a process can be conducted at elevated temperatures, for example a temperature of about 30° C., about 40° C., about 50° C., about 60° C., about 70° C., or about 80° C.
Generally, a reaction is controlled or a post-processing of a particle(s) is carried out to obtain a core where polythionate is increased up to about 25 wt. % of the core, up to about 20 wt. %, up to about 15 wt. %, or up to about 10 wt. %. By controlling a reaction, it is possible to increase wt. % of polythionate that forms or is retained within a core. In some embodiments, for example, where too much polythionate is present, it is possible to reduce total amount of polythionate by aging a sulfur-based particle. In some embodiments, composition of a core can be measured via, for example, high-pressure liquid chromatography (HPLC), ion chromatography (IC), combustion analysis or by thermogravimetric analysis (TGA). Other means of determining composition of a core are disclosed in The Molecular Composition of Hydrophillic Sulfur Sols Prepared by Acid Decomposition of Thiosulfate, Steudel et al, Z. Naturforsch 43b, 203-218 (1988), the entire disclosure of which is incorporated by reference herein.
During charging, specifically recharging, lithium ions 30 move back through electrolyte 16 towards anode 24, and electrons 28 travel back through external circuit 32. Typically, lithium ions 30 react with any degradation products (e.g., cathode active materials that have dissolved into electrolyte, such as polysulfides) in electrolyte 16 forming insoluble solids that lead to anode and cathode fouling and reduced capacity and slower charging. Using nanoparticles described herein helps to retain sulfur in a cathode to reduce or eliminate formation of polysulfides in electrolyte 16. Without wishing to be bound by any particular theory, it is believed that this is due, at least in part, to the fact that polythionates do not degrade.
It is contemplated that compositions of matter and processes of present application encompass variations and adaptations developed using information from embodiments described in the present disclosure. Adaptation or modification of methods and processes described in this specification may be performed by those of ordinary skill in the relevant art.
Throughout the description, where compositions, compounds, or products are described as having, including, or comprising specific components, or where processes and methods are described as having, including, or comprising specific steps, it is contemplated that, additionally, there are articles, devices, and systems of the present application that consist essentially of, or consist of, the recited components, and that there are processes and methods according to the present application that consist essentially of, or consist of, the recited processing steps.
It should be understood that order of steps or order for performing certain action is immaterial so long as a described method remains operable. Moreover, two or more steps or actions may be conducted simultaneously.
This application claims priority to and benefit of U.S. Application Ser. No. 62/823,765, filed on Mar. 26, 2019, the contents of which is incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2020/024725 | 3/25/2020 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62823765 | Mar 2019 | US |
