Patent Application
20210151741
The present disclosure relates generally to the field of alkali metal battery or alkali metal-ion battery and, more particularly, to a lithium or sodium secondary battery anode having multiple graphene balls, each comprising multiple graphene sheets having a lithium- or sodium-attracting metal supported thereon, and a process for producing the graphene balls, the electrode and the battery.
Rechargeable lithium-ion (Li-ion) and lithium metal batteries (e.g. Li-sulfur, Li metal-air, and lithium-metal oxide batteries) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium as a metal element has the highest capacity (3,861 mAh/g) compared to any other metal. Hence, in general, Li metal batteries have a significantly higher energy density than lithium ion batteries. Similarly, Na metal batteries have a higher energy than corresponding sodium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds, such as TiS2, MoS2, MnO2, CoO2, and V2O5, as the cathode active materials, coupled with a lithium metal anode. When the battery was discharged, lithium ions were transferred from the lithium metal anode through the electrolyte to the cathode, and the cathode became lithiated. Unfortunately, upon repeated charges/discharges, the lithium metal resulted in the formation of dendrites at the anode that ultimately grew to penetrate through the separator, causing internal shorting and explosion. As a result of a series of accidents associated with this problem, the production of these types of secondary batteries was stopped in the early 1990's.
To overcome these safety issues, several alternative approaches were proposed in which either the electrolyte or the anode was modified. The first approach involves replacing Li metal by graphite (a Li insertion material) as the anode. The operation of such a battery involves shuttling Li ions between two Li insertion compounds at the anode and the cathode, respectively; hence, the name “Li-ion battery.” Presumably because of the presence of Li in its ionic rather than metallic state, Li-ion batteries are inherently safer than Li-metal batteries. The second approach entails replacing the liquid electrolyte by a dry polymer electrolyte, leading to the Li solid polymer electrolyte (Li-SPE) batteries. However, Li-SPE has seen very limited applications since it typically requires an operating temperature of up to 80° C. The third approach involves the use of a solid electrolyte that is presumably resistant to dendrite penetration, but the solid electrolyte normally exhibits excessively low lithium-ion conductivity at room temperature. Alternative to this solid electrolyte approach is the use of a rigid solid protective layer between the anode active material layer and the separator layer to stop dendrite penetration, but this typically ceramic material-based layer also has a low ion conductivity and is difficult and expensive to make and to implement in a lithium metal battery. Furthermore, the implementation of such a rigid and brittle layer is incompatible with the current lithium battery manufacturing process and equipment.
Although lithium-ion (Li-ion) batteries are promising energy storage devices for electric drive vehicles, state-of-the-art Li-ion batteries have yet to meet the cost and performance targets. Li-ion cells typically use a lithium transition-metal oxide or phosphate as a positive electrode (cathode) that de/re-intercalates Li+ at a high potential with respect to the carbon negative electrode (anode). The specific capacity of graphite anode is <372 mAh/g and that of lithium transition-metal oxide or phosphate based cathode active material is typically in the range from 140-220 mAh/g. As a result, the specific energy of commercially available Li-ion cells is typically in the range from 120-240 Wh/kg, most typically 150-220 Wh/kg. These specific energy values are significantly lower than what would be required for battery-powered electric vehicles to be widely accepted.
With the rapid development of hybrid (HEV), plug-in hybrid electric vehicles (HEV), and all-battery electric vehicles (EV), there is an urgent need for anode and cathode materials that provide a rechargeable battery with a significantly higher specific energy, higher energy density, higher rate capability, long cycle life, and safety. Among various advanced energy storage devices, alkali metal batteries, including Li-air (or Li—O2), Na-air (or Na—O2), Li—S, and Na—S batteries, are especially attractive due to their high specific energies.
The Li—O2 battery is possibly the highest energy density electrochemical cell that can be configured today. The Li—O2 cell has a theoretic energy density of 5.2 kWh/kg when oxygen mass is accounted for. A well configured Li—O2 battery can achieve an energy density of 3,000 Wh/kg, 15-20 times greater than those of Li-ion batteries. However, current Li—O2 batteries still suffer from poor energy efficiency, poor cycle efficiency, and dendrite formation and penetration issues.
One of the most promising energy storage devices is the lithium metal anode based battery, such as lithium-sulfur (Li—S) cell, since the theoretical capacity of Li is 3,861 mAh/g and that of S is 1,675 mAh/g. In its simplest form, a Li—S cell consists of elemental sulfur as the positive electrode and lithium as the negative electrode. The lithium-sulfur cell operates with a redox couple, described by the reaction S8+16Li⇄8Li2S that lies near 2.2 V with respect to Li+/Li∘. This electrochemical potential is approximately ⅔ of that exhibited by conventional positive electrodes (e.g. LiMnO4). However, this shortcoming is offset by the very high theoretical capacities of both Li and S. Thus, compared with conventional intercalation-based Li-ion batteries, Li—S cells have the opportunity to provide a significantly higher energy density (a product of capacity and voltage). Assuming complete reaction to Li2S, energy densities values can approach 2,500 Wh/kg and 2,800 Wh/l, respectively, based on the combined Li and S weights or volumes. If based on the total cell weight or volume, the energy densities can reach approximately 1,000 Wh/kg and 1,100 Wh/l, respectively. However, the current Li-sulfur cells reported by industry leaders in sulfur cathode technology have a maximum cell specific energy of 250-350 Wh/kg (based on the total cell weight), which is far below what is possible.
In summary, despite its great potential, the practical realization of the Li—S battery has been hindered by several obstacles, such as dendrite-induced internal shorting, low active material utilization efficiency, high internal resistance, self-discharge, and rapid capacity fading on cycling. These technical barriers are due to the poor electrical conductivity of elemental sulfur, the high solubility of lithium polysulfides in organic electrolyte (which migrate to the anode side, resulting in the formation of inactivated Li2S in the anode), and Li dendrite formation and penetration. The most serious problems of Li metal secondary (rechargeable) batteries (including all sorts of cathode active materials, such as S, Se, NCM, NCM, other lithium transition metal oxide, sodium-transition metal oxide, etc.) remains to be the dendrite formation and penetration, high solid-electrolyte interfacial impedance, and poor cycle life. Sodium metal batteries have similar problems to address.
Furthermore, it has been challenging and expensive to deposit or attach a layer of lithium metal (or sodium metal) on surfaces of an anode current collector (Cu foil). There is a need to reduce the amount of lithium metal or sodium metal in the anode of a lithium metal or sodium metal battery. It would be desirable and preferable if the presence of a lithium or sodium metal layer (film, foil, or coating) is eliminated when the cell is made. The lithium metal or sodium metal is then supplied from the cathode side (e.g. lithium transition metal oxide or sodium transition metal oxide) during the subsequent battery charging operations.
It is an object of the present disclosure to overcome most of the afore-mentioned problems associated with current lithium metal batteries or sodium metal batteries. A specific object of the present disclosure is to provide graphene balls or particulates (including both lithium- or sodium-loaded balls and those graphene balls without lithium or sodium pre-loaded therein) for use as an anode active material for lithium metal and sodium metal secondary batteries that exhibit long and stable charge-discharge cycle life without exhibiting lithium or sodium dendrite problems.
The present disclosure provides powder mass, comprising multiple graphene balls that contain a metal supported on graphene sheets in these balls, as an anode material for an alkali metal battery (lithium or sodium metal battery) and a process for producing such powder mass. The disclosure also provides a lithium metal battery and a sodium metal containing such graphene balls as an anode active material.
In certain embodiments, the disclosure provides a powder mass comprising multiple metal-containing graphene balls or particulates as an anode active material for a lithium battery or sodium battery, the graphene ball or particulate comprising (a) a plurality of graphene sheets, each having a length or width preferably from 5 nm to 100 μm and forming into the ball or particulate having a diameter from 100 nm to 20 μm and (b) a lithium-attracting metal or sodium-attracting metal in a form of particles or coating having a diameter or thickness from 0.5 nm to 10 μm and in physical contact with the graphene sheets, wherein the metal is selected from Au, Ag, Mg, Zn, Ti, Na, K, Al, Fe, Mn, Co, Ni, Sn, V, Cr, an alloy thereof, or a combination thereof and is in an amount of 0.1% to 95% of the total particulate weight (more typically from 0.1% to 30%). The graphene ball or particulate can be substantially spherical, ellipsoidal, slightly elongated, or irregular in shape.
The graphene sheets contain single-layer or few-layer graphene, wherein the few-layer graphene sheets have 2-10 layers of stacked graphene planes having an inter-plane spacing d002 from 0.3354 nm to 0.60 nm as measured by X-ray diffraction and the single-layer or few-layer graphene sheets contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.001% to 25% by weight of non-carbon elements.
The non-pristine graphene may be selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, doped graphene, chemically functionalized graphene, or a combination thereof.
In certain embodiments, the graphene ball or particulate further comprises therein an adhesive or an electron-conducting or ion-conducting material (lithium ion-conducting or sodium ion-conducting) as a binder or matrix material that helps to hold multiple graphene sheets in a ball together, if so desired. The electron-conducting material may be selected from an intrinsically conducting polymer, a carbon, a pitch material, a metal, or a combination thereof, wherein the metal (as a conductive additive or binder) can include or not include Au, Ag, Mg, Zn, Ti, Li, Na, K, Al, Fe, Mn, Co, Ni, Sn, V, Cr, or an alloy thereof.
The intrinsically conducting polymer is preferably selected from (but not limited to) polyaniline, polypyrrole, polythiophene, polyfuran, polyacetylene, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof.
The lithium ion-conducting material may be selected from Li2CO3, Li2O, Li2C2O4, LiOH, LiX, ROCO2Li, HCOLi, ROLi, (ROCO2Li)2, (CH2OCO2Li)2, Li2S, LixSOy, or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.
In certain embodiments, the lithium ion-conducting material in the encapsulating shell contains a lithium salt selected from lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium borofluoride (LiBF4), lithium hexafluoroarsenide (LiAsF6), lithium trifluoro-methanesulfonate (LiCF3SO3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF3SO2)2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF2C2O4), lithium nitrate (LiNO3), Li-fluoroalkyl-phosphate (LiPF3(CF2CF3)3), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof.
In some embodiments, the ion-conducting material comprises a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, or a combination thereof.
In some embodiments, the lithium ion-conducting material in the graphene ball comprises a sulfonated polymer, which is typically conductive to lithium ions or sodium ions.
The graphene ball or particulate may further contain an electron-conducting material, disposed therein, selected from expanded graphite flake, carbon nanotube, carbon nano-fiber, carbon fiber, carbon particle, graphite particle, carbon black, acetylene black, pitch, an electron-conducting polymer, or a combination thereof. The electron-conducting polymer may be selected from (but not limited to) polyaniline, polypyrrole, polythiophene, polyfuran, polyacetylene, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof. Any intrinsically conductive polymer may be used for this purpose.
In some preferred embodiments, the graphene particulate further contains lithium metal or sodium metal (particles or coating) residing inside the particulate and in physical contact with the lithium-attracting metal or sodium-attracting metal to form a lithium-preloaded or sodium-preloaded graphene particulate.
The particulate is preferably pre-loaded with lithium or sodium metal (impregnated into the core of the particulate) before the battery is made. Alternatively, the anode of the intended alkali metal battery contains a lithium source or a sodium source, in addition to the graphene particulates or balls. The lithium source is preferably selected from foil, particles, or filaments of lithium metal or lithium alloy having no less than 80% by weight of lithium element in the lithium alloy. The sodium source is preferably selected from foil, particles, or filaments of sodium metal or sodium alloy having no less than 80% by weight of sodium element in the sodium alloy.
In the lithium or sodium metal battery, each cell contains an anode layer comprising the disclosed graphene particulates or balls, which are pre-loaded with lithium or sodium. When the battery is discharged, lithium or sodium ions are released from the particulates and moved through an electrolyte/separator to the cathode comprising a cathode active material layer. The resulting particulates will accommodate lithium or sodium when the battery is subsequently recharged. In some embodiments, the lithium or sodium metal battery further comprises a separator, discrete anode current collector (e.g. Cu foil) in contact with the anode. Typically, there is a separate, discrete cathode current collector (e.g. Al foil) in contact with the cathode active material layer (containing cathode active material, such as MoS2, TiO2, V2O5, LiV3O8, S, Se, NCM, NCA, or other lithium transition metal oxides, etc.), which is supported by (coated on) the Al foil.
In some embodiments, the anode of the lithium cell or sodium cell comprises the presently disclosed powder mass of graphene balls but without the presence of a lithium or sodium metal layer (no particle, film, foil, or coating of Li or Na metal) when the cell is made. The lithium metal or sodium metal is then supplied from the cathode side (e.g. lithium transition metal oxide or sodium transition metal oxide) during the first and subsequent battery charging operations. This avoids the need to deal with lithium metal or sodium metal (highly sensitive to oxygen and moisture in the room air) during battery fabrication. It is challenging and expensive to handle lithium or sodium metal in a manufacturing facility.
In some embodiments, the graphene particulate, when measured without the lithium- or sodium-attracting metal, has a density from 0.05 to 1.7 g/cm3 and a specific surface area from 50 to 2,630 m2/g. In certain embodiments, the particulate, when measured without the metal, has a density from 0.1 to 1.7 g/cm3 and has some pores with an average pore size from 10 nm to 10 μm. In some embodiments, the particulate has a physical density higher than 0.8 g/cm3 and a specific surface area greater than 600 m2/g. In some embodiments, the graphene particulate has a physical density higher than 1.0 g/cm3 and a specific surface area greater than 300 m2/g.
The graphene in the particulate may comprise a non-pristine graphene material having a content of non-carbon elements in the range from 0.01% to 20% by weight and the non-carbon elements include an element selected from oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or boron.
The disclosure also provides an alkali metal battery anode containing a plurality of the invented particulates as an anode active material. In certain embodiments, the alkali metal battery comprises a cathode, an anode containing the disclosed graphene particulates or graphene balls, an optional lithium source or a sodium source in ionic contact with the anode, and an electrolyte in ionic contact with both the cathode and the anode. The lithium source may be selected from foil, particles, or filaments of lithium metal or lithium alloy having no less than 80% by weight of lithium element in the lithium alloy; or the sodium source is selected from foil, particles, or filaments of sodium metal or sodium alloy having no less than 80% by weight of sodium element in the sodium alloy.
The disclosure also provides (a) an alkali metal battery anode containing a plurality of the presently disclosed graphene balls that are preloaded with lithium metal or sodium metal as an anode active material and (b) an alkali metal battery comprising such an anode, a cathode, and an electrolyte in ionic contact with both the cathode and the anode.
The alkali metal battery may be a lithium metal battery, lithium-sulfur battery, lithium-selenium battery, lithium-air battery, sodium metal battery, sodium-sulfur battery, sodium-selenium battery, or sodium-air battery.
The disclosure also provides a lithium-ion battery or sodium-ion battery comprising an anode, a cathode, an electrolyte in ionic contact with the anode and the cathode, wherein the anode comprises a first anode active material, comprising a plurality of the invented lithium-preloaded graphene particulates, and a second anode active material, wherein the lithium-preloaded graphene particulates act as a lithium source for the second anode active material when an electrolyte is introduced into such an anode (comprising the two types of anode active material) or during a charge/discharge cycle of the lithium-ion battery. The lithium-preloaded graphene particulates act to pre-lithiate the second (or primary) anode active material. In other words, the presently disclosed lithium-preloaded graphene particulates can serve as a pre-lithiating agent for any anode active material in a conventional lithium-ion battery.
In the above-described lithium-ion battery, the second anode active material may be selected from the group consisting of: (A) silicon (Si), phosphorus (P), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (B) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (C) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, or Cd, and their mixtures, composites, or lithium-containing composites; (D) salts and hydroxides of Sn; (E) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; (F) graphite or carbon particles, filaments, fibers, nano-fibers, nano-tubes, or nano-wires; and combinations thereof.
The disclosure also provides a sodium-ion battery comprising an anode, a cathode, an electrolyte in ionic contact with the anode and the cathode, wherein the anode comprises a first anode active material, comprising a plurality of the sodium-preloaded graphene particulates, and a second anode active material, wherein the sodium-preloaded graphene particulates act as a sodium source for the second anode active material when an electrolyte is introduce into such an anode or during a charge/discharge cycle of the sodium-ion battery. In such a sodium-ion battery, the second anode active material is selected from the group consisting of: (a) silicon (Si), phosphorus (P), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), nickel (Ni), cobalt (Co), and cadmium (Cd); (b) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, or Cd with other elements; (c) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Fe, Ni, Co, or Cd, and their mixtures, composites, or lithium-containing composites; (d) graphite or carbon particles, filaments, fibers, nano-fibers, nano-tubes, or nano-wires; and (e) combinations thereof.
Thus, the disclosure further provides a method of pre-lithiating or pre-sodiating a lithium-ion battery or sodium-ion battery, the method comprising an operation of combining lithium-preloaded or sodium-preloaded graphene particulates, as a first anode active material, and a second anode active material in an anode of a lithium-ion battery or sodium-ion battery and introducing an electrolyte into the anode. This step of introducing electrolyte into the anode may be accomplished before or after such an anode is incorporated with a cathode and a separator to form a battery cell.
Also provided in the disclosure is a process for producing a powder mass of graphene balls or particulates for an alkali metal battery, the process comprising:
Step (a) of combining may include a procedure of depositing particles or coating of the lithium-attracting metal or sodium-attracting metal onto surfaces of the multiple graphene sheets to obtain the graphene/metal mixture, which comprises multiple metal-deposited graphene sheets.
Decoration or deposition of a Li- or Na-attracting metal onto surfaces of graphene sheets prior to being subjected to graphene ball formation may be accomplished via using various depositing or coating means (e.g. melt dipping, solution deposition, chemical vapor deposition, physical vapor deposition, sputtering, electrochemical deposition, etc.).
For the purpose of defining the scope of the claims, the lithium- or sodium-attracting metal recited in Step (a) includes a precursor to this metal; such a precursor may be later chemically or thermally converted to the desired metal. For instance, graphene surfaces may be coated with HAuCl4, which is then thermally converted to Au when the graphene balls are heated. Another example is to deposit zinc chloride on graphene surfaces (e.g. via salt solution dipping and drying) and use hydrogen and methane to chemically convert this precursor to Zn metal at a later stage (e.g. before or after graphene deposition). There are many metal precursors to metals that are well-known in the art.
In some embodiments, the process may include (a) depositing a metal-containing precursor (e.g. an organo-metallic molecule or a metal salt) onto the surfaces of multiple graphene sheets to form precursor-coated graphene sheets or mixing a metal-containing precursor with multiple graphene sheets to form metal precursor/graphene mixture; (b) forming the precursor-coated graphene sheets or metal precursor/graphene mixture into graphene balls containing metal precursor therein; and (c) heat treating the graphene balls to thermally convert or chemically treating the graphene balls to chemically reduce the metal precursor to a metal phase, wherein the metal resides in the pores of the resulting particulates or adheres to graphene sheet surfaces in the particulates.
Step (b) of forming graphene balls may be conducted via several procedures. Two examples are given below:
In the aforementioned (a), the ball milling procedure is preferably conducted by using an energy impacting apparatus selected from a double cone mixer, double cone blender, vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, cryogenic ball mill, micro ball mill, tumbler ball mill, attritor, continuous ball mill, stirred ball mill, pressurized ball mill, plasma-assisted ball mill, freezer mill, vibratory sieve, bead mill, nano bead mill, ultrasonic homogenizer mill, centrifugal planetary mixer, vacuum ball mill, or resonant acoustic mixer. In certain preferred embodiments, the procedure of operating the energy impacting apparatus is conducted in a continuous manner using a continuous energy impacting device.
The milling media may be selected from particles of a metal or metal alloy, a glass, a ceramic, a polymer, or a combination thereof.
The disclosed process may further comprise a step of impregnating lithium metal or sodium metal into the graphene particulates, wherein the lithium metal or sodium metal partially or completely fills the pore (if any) and is in physical contact with the lithium-attracting metal or sodium-attracting metal to form lithium-preloaded or sodium-preloaded graphene particulates.
The process may further comprise a step of incorporating the graphene particulates in an electrode for a lithium metal battery, lithium-sulfur battery, lithium-selenium battery, lithium-air battery, sodium metal battery, sodium-sulfur battery, sodium-selenium battery, or sodium-air battery.
In certain embodiments, the process may further comprise a step of incorporating the lithium-preloaded or sodium-preloaded graphene particulates in an anode electrode as a pre-lithiating agent or a pre-sodiating agent for a lithium metal battery, lithium-sulfur battery, lithium-selenium battery, lithium-air battery, sodium metal battery, sodium-sulfur battery, sodium-selenium battery, or sodium-air battery.
In certain embodiments, Step (a) of the invented process comprises depositing a precursor to lithium-attracting metal or sodium-attracting metal onto surfaces of graphene sheets and Step (e) comprises thermally converting the precursor to the lithium-attracting metal or sodium-attracting metal.
In certain embodiments, Step (a) comprises depositing a precursor to lithium-attracting metal or sodium-attracting metal onto surfaces of graphene sheets and then chemically or thermally converting the precursor to the lithium-attracting metal or sodium-attracting metal.
As schematically illustrated in
The prior art lithium or sodium metal cell is typically made by a process that includes the following steps: (a) The first step is mixing and dispersing particles of the cathode active material (e.g. activated carbon), a conductive filler (e.g. acetylene black), a resin binder (e.g. PVDF) in a solvent (e.g. NMP) to form a cathode slurry; (b) The second step includes coating the cathode slurry on the surface(s) of an Al foil and drying the slurry to form a dried cathode electrode coated on the Al foil; (c) The third step includes laminating a Cu foil (as an anode current collector), a sheet of Li or Na foil (or lithium alloy or sodium alloy foil), a porous separator layer, and a cathode electrode-coated Al foil sheet together to form a 5-layer assembly, which is cut and slit into desired sizes and stacked to form a rectangular structure (as an example of shape) or rolled into a cylindrical cell structure; (d) The rectangular or cylindrical laminated structure is then encased in an aluminum-plastic laminated envelope or steel casing; and (e) A liquid electrolyte is then injected into the laminated structure to make a lithium battery cell.
Due to the high specific capacity of lithium metal and sodium metal, the highest battery energy density can be achieved by alkali metal rechargeable batteries that utilize a lithium metal or sodium metal as the anode active material, provided that a solution to the safety problem can be formulated. These cells include (a) the traditional Li or Na metal battery having a Li insertion or Na insertion compound in the cathode, (b) the Li-air or Na—O2 cell that uses oxygen, instead of metal oxide, as a cathode (and Li or sodium metal as an anode instead of graphite or hard carbon particles), (c) the Li-sulfur, Na—S, or other cell using a conversion-type cathode active material, and (d) the lithium-selenium cell or sodium-selenium cell.
The Li—O2 battery is possibly the highest energy density electrochemical cell that can be configured today. The Li—O2 cell has a theoretic energy density of 5,200 Wh/kg when oxygen mass is accounted for. A well configured Li—O2 battery can achieve an energy density of 3,000 Wh/kg, which is 15-20 times greater than those of Li-ion batteries. However, current Li—O2 batteries still suffer from poor energy efficiency, poor cycle efficiency, and dendrite formation issues.
In the Li—S cell, elemental sulfur (S) as a cathode material exhibits a high theoretical Li storage capacity of 1,672 mAh/g. With a Li metal anode, the Li—S battery has a theoretical energy density of ˜1,600 Wh/kg (per total weight of active materials). Despite its great potential, the practical realization of the Li—S battery has been hindered by several obstacles, such as low utilization of active material, high internal resistance, self-discharge, and rapid capacity fading on cycling. These technical barriers are due to the poor electrical conductivity of elemental sulfur, the high solubility of lithium polysulfides in organic electrolyte, the formation of inactivated Li2S, the formation of Li dendrites on the anode, and high solid-electrolyte interfacial impedance at the anode. Despite great efforts worldwide, dendrite formation and high interfacial impedance remain the most critical scientific and technological barriers against widespread implementation of all kinds of high energy density batteries having a Li metal anode. The same problems have also prevented commercial application of sodium metal batteries.
We have discovered a highly dendrite-resistant or dendrite-free, graphene ball-based anode configuration for a Li metal cell or Na metal cell that exhibits a high energy, high power density, and stable cycling behavior. In certain embodiments, the disclosure provides a powder mass comprising multiple metal-containing graphene balls or particulates as an anode active material for a lithium battery or sodium battery, the graphene ball or particulate (as illustrated in
The lithium- or sodium-attracting metal material can contain a metal (M) selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, Pd, Ag, Cd, Au, Pt, W, Al, Sn, In, Pb, Bi, Na, Li, Mg, Ca, an alloy thereof, or a mixture thereof. These elements have the characteristics that (a) the element itself or its alloy with another metal element can alloy with lithium or sodium ions at a temperature from −50° C. to +100° C. (capable of forming LiMx, NaMx, LiMayMbz, or NaMayMbz, where x, y, or z is from 0.01 to 6) when these ions return from the cathode during the battery charging operation; or (b) the surfaces of these elements or their alloy with another metal element can be wetted by lithium ions or sodium ions. Most of the transition metals or alkaline metals can be used, but preferably, the metal is selected from Zn, Al, Ag, Au, Ti, Sn, Fe, Mg, Cu, or an alloy thereof with another metal.
The terms “graphene particulates” and “graphene balls” are herein used interchangeably.
Schematically shown in
The graphene balls or particulates can be lithiated (loaded with Li) or sodiated (loaded with Na) before or after the cell is made. For instance, when the cell is made, a foil or particles of lithium or sodium metal (or metal alloy) may be implemented at the anode (e.g. between a layer of multiple graphene particulates and a porous separator or solid electrolyte) to supply this layer of graphene particulates with lithium or sodium. This lithiation or sodiation procedure can occur when the lithium or sodium foil layer is in close contact with the layer of graphene particulates and a liquid electrolyte is introduced into the anode or the entire cell.
Additionally, during the first battery discharge cycle, lithium (or sodium) is ionized, supplying lithium (or sodium) ions (Li+ or Na+) into electrolyte. These Li+ or Na+ ions migrate to the cathode side and get captured by and stored in the cathode active material (e.g. vanadium oxide, MoS2, S, etc.). During the subsequent re-charge cycle of the battery, Li+ or Na+ ions are released by the cathode active material and migrate back to the anode. These Li+ or Na+ ions naturally diffuse into the graphene balls to reach the lithium- or sodium-attracting metal lodged inside the graphene particulates. In this manner, the particulates are said to be lithiated or sodiated.
Alternatively, the graphene particulates (or balls) can be lithiated or sodiated (herein referred to as “pre-lithiated” or “pre-sodiated”) electrochemically prior to being incorporated as an anode layer into the cell structure. This can be accomplished by bringing a mass of graphene balls in contact with a lithium or sodium foil in the presence of a liquid electrolyte, or by implementing a layer of graphene balls as a working electrode and a lithium/sodium foil or rod as a counter-electrode in an electrochemical reactor chamber containing a liquid electrolyte. By introducing an electric current between the working electrode and the counter-electrode, one can introduce lithium or sodium into the graphene particulates, wherein Li+ or Na+ ions diffuse into the pores of the particulates to initially form a lithium or sodium alloy with the lithium- or sodium-attracting metal pre-lodged therein. Presumably, such an initially or previously formed alloy can act as a buffer zone or as a heterogeneous nucleating seed to promote growth of lithium or sodium metal in the pores. Without the inclusion of a lithium- or sodium-attracting metal inside the graphene balls, some lithium or sodium metal can get deposited on exterior surfaces of the graphene particulates.
Graphene is a single-atom thick layer of sp2 carbon atoms arranged in a honeycomb-like lattice. Graphene can be readily prepared from graphite, activated carbon, graphite fibers, carbon black, and meso-phase carbon beads. Single-layer graphene and its slightly oxidized version (GO) can have a specific surface area (SSA) as high as 2630 m2/g. It is this high surface area that dramatically reduces the effective electrode current density, which in turn significantly reduces or eliminates the possibility of Li dendrite formation. However, we have unexpectedly observed that it is difficult for the returning lithium ions or sodium ions (those that return from the cathode back to the anode during battery charge) to uniformly deposit to graphene sheets and well-adhere to these graphene sheets in a porous graphene structure alone without the presence of a lithium- or sodium-attracting metal. Lithium or sodium has a high tendency to not adhere well to graphene surfaces or to get detached therefrom, thereby becoming isolated lithium or sodium clusters that no longer participate in reversible lithium/sodium storage. We have further surprisingly observed that such a lithium-or sodium-attracting metal, if present on the internal graphene surface or residing in pores of a graphene particulate, provides a safe and reliable site to receive and accommodate lithium/sodium during the battery charging step. The resulting lithium alloy or sodium alloy is also capable of reversibly releasing lithium or sodium ions into electrolyte that travel to the cathode side during the subsequent battery discharging step.
Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube or carbon nano-fiber (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of a few nanometers to a few hundred nanometers. Their longitudinal, hollow structures impart unique mechanical, electrical and chemical properties to the material. The CNT or CNF is a one-dimensional nano carbon or 1-D nano graphite material.
A single-layer graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Multi-layer graphene is a platelet composed of more than one graphene plane. Individual single-layer graphene sheets and multi-layer graphene platelets are herein collectively called nano graphene platelets (NGPs) or graphene materials. NGPs include pristine graphene (essentially 99% of carbon atoms), slightly oxidized graphene (<5% by weight of oxygen), graphene oxide (≥5% by weight of oxygen), slightly fluorinated graphene (<5% by weight of fluorine), graphene fluoride ((≥5% by weight of fluorine), other halogenated graphene, and chemically functionalized graphene.
Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004) (U.S. Pat. Pub. No. 2005/0271574) (now abandoned); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006) (U.S. Pat. Pub. No. 2008/0048152) (now abandoned).
Our research group also presented the first review article on various processes for producing NGPs and NGP nanocomposites [Bor Z. Jang and A Zhamu, “Processing of Nano Graphene Platelets (NGPs) and NGP Nanocomposites: A Review,” J. Materials Sci. 43 (2008) 5092-5101]. Four main prior-art approaches have been followed to produce NGPs. The most commonly used process is chemical oxidation and reduction of graphite to produce graphene oxide (GO) and reduced graphene oxide (RGO).
This process, as schematically illustrated in
In the thermal shock exposure approach, the GIC or GO is exposed to a high temperature (typically 800-1,050° C.) for a short period of time (typically 15 to 60 seconds) to exfoliate or expand the GIC or GO for the formation of exfoliated or further expanded graphite, which is typically in the form of a “graphite worm” composed of graphite flakes that are still interconnected with one another. This thermal shock procedure can produce some separated graphite flakes or graphene sheets, but normally the majority of graphite flakes remain interconnected. Typically, the exfoliated graphite or graphite worm is then subjected to a flake separation treatment using air milling, mechanical shearing, or ultrasonication in water. Hence, approach 1 basically entails three distinct procedures: first expansion (oxidation or intercalation), further expansion (or “exfoliation”), and separation.
In the solution-based separation approach, the expanded but un-exfoliated or exfoliated GO powder is dispersed in water or aqueous alcohol solution, which is subjected to ultrasonication. It is important to note that in these processes, ultrasonification is used after intercalation and oxidation of graphite (i.e., after first expansion) and can be after thermal shock exposure of the resulting GIC or GO (after second expansion). Alternatively, the GO powder dispersed in water is subjected to an ion exchange or lengthy purification procedure in such a manner that the repulsive forces between ions residing in the inter-planar spaces overcome the inter-graphene van der Waals forces, resulting in graphene layer separations.
In the aforementioned examples, the starting material for the preparation of graphene sheets or NGPs is a graphitic material that may be selected from the group consisting of natural graphite, artificial graphite, graphite oxide, graphite fluoride, graphite fiber, carbon fiber, carbon nano-fiber, carbon nano-tube, mesophase carbon micro-bead (MCMB) or carbonaceous micro-sphere (CMS), soft carbon, hard carbon, and combinations thereof.
Graphite oxide may be prepared by dispersing or immersing a laminar graphite material (e.g., powder of natural flake graphite or synthetic graphite) in an oxidizing agent, typically a mixture of an intercalant (e.g., concentrated sulfuric acid) and an oxidant (e.g., nitric acid, hydrogen peroxide, sodium perchlorate, potassium permanganate) at a desired temperature (typically 0-70° C.) for a sufficient length of time (typically 4 hours to 5 days). The resulting graphite oxide particles are then rinsed with water several times to adjust the pH values to typically 2-5. The resulting suspension of graphite oxide particles dispersed in water is then subjected to ultrasonication to produce a dispersion of separate graphene oxide sheets dispersed in water. A small amount of reducing agent (e.g. Na4B) may be added to obtain reduced graphene oxide (RGO) sheets.
In order to reduce the time required to produce a precursor solution or suspension, one may choose to oxidize the graphite to some extent for a shorter period of time (e.g., 30 minutes-4 hours) to obtain graphite intercalation compound (GIC). The GIC particles are then exposed to a thermal shock, preferably in a temperature range of 600-1,100° C. for typically 15 to 60 seconds to obtain exfoliated graphite or graphite worms, which are optionally (but preferably) subjected to mechanical shearing (e.g. using a mechanical shearing machine or an ultrasonicator) to break up the graphite flakes that constitute a graphite worm. Either the already separated graphene sheets (after mechanical shearing) or the un-broken graphite worms or individual graphite flakes are then re-dispersed in water, acid, or organic solvent and ultrasonicated to obtain a graphene dispersion.
The pristine graphene material is preferably produced by one of the following three processes: (A) Intercalating the graphitic material with a non-oxidizing agent, followed by a thermal or chemical exfoliation treatment in a non-oxidizing environment; (B) Subjecting the graphitic material to a supercritical fluid environment for inter-graphene layer penetration and exfoliation; or (C) Dispersing the graphitic material in a powder form to an aqueous solution containing a surfactant or dispersing agent to obtain a suspension and subjecting the suspension to direct ultrasonication to obtain a graphene dispersion.
In Procedure (A), a particularly preferred step comprises (i) intercalating the graphitic material with a non-oxidizing agent, selected from an alkali metal (e.g., potassium, sodium, lithium, or cesium), alkaline earth metal, or an alloy, mixture, or eutectic of an alkali or alkaline metal; and (ii) a chemical exfoliation treatment (e.g., by immersing potassium-intercalated graphite in ethanol solution).
In Procedure (B), a preferred step comprises immersing the graphitic material to a supercritical fluid, such as carbon dioxide (e.g., at temperature T>31° C. and pressure P>7.4 MPa) and water (e.g., at T>374° C. and P>22.1 MPa), for a period of time sufficient for inter-graphene layer penetration (tentative intercalation). This step is then followed by a sudden de-pressurization to exfoliate individual graphene layers. Other suitable supercritical fluids include methane, ethane, ethylene, hydrogen peroxide, ozone, water oxidation (water containing a high concentration of dissolved oxygen), or a mixture thereof.
In Procedure (C), a preferred step comprises (a) dispersing particles of a graphitic material in a liquid medium containing therein a surfactant or dispersing agent to obtain a suspension or slurry; and (b) exposing the suspension or slurry to ultrasonic waves (a process commonly referred to as ultrasonication) at an energy level for a sufficient length of time to produce a graphene dispersion of separated graphene sheets (non-oxidized NGPs) dispersed in a liquid medium (e.g. water, alcohol, or organic solvent).
Graphene materials can be produced with an oxygen content no greater than 25% by weight, preferably below 20% by weight, further preferably below 5%. Typically, the oxygen content is between 5% and 20% by weight. The oxygen content can be determined using chemical elemental analysis and/or X-ray photoelectron spectroscopy (XPS). When the oxygen content of graphene oxide exceeds 30% by weight (more typically when >35%), the GO molecules dispersed or dissolved in water for a GO gel state.
The laminar graphite materials used in the prior art processes for the production of the GIC, graphite oxide, and subsequently made exfoliated graphite, flexible graphite sheets, and graphene platelets were, in most cases, natural graphite. However, the present disclosure is not limited to natural graphite. The starting material may be selected from the group consisting of natural graphite, artificial graphite (e.g., highly oriented pyrolytic graphite, HOPG), graphite oxide, graphite fluoride, graphite fiber, carbon fiber, carbon nano-fiber, carbon nano-tube, mesophase carbon micro-bead (MCMB) or carbonaceous micro-sphere (CMS), soft carbon, hard carbon, and combinations thereof. All of these materials contain graphite crystallites that are composed of layers of graphene planes stacked or bonded together via van der Waals forces. In natural graphite, multiple stacks of graphene planes, with the graphene plane orientation varying from stack to stack, are clustered together. In carbon fibers, the graphene planes are usually oriented along a preferred direction. Generally speaking, soft carbons are carbonaceous materials obtained from carbonization of liquid-state, aromatic molecules. Their aromatic ring or graphene structures are more or less parallel to one another, enabling further graphitization. Hard carbons are carbonaceous materials obtained from aromatic solid materials (e.g., polymers, such as phenolic resin and polyfurfuryl alcohol). Their graphene structures are relatively randomly oriented and, hence, further graphitization is difficult to achieve even at a temperature higher than 2,500° C. But, graphene sheets do exist in these carbons.
Fluorinated graphene or graphene fluoride is herein used as an example of the halogenated graphene material group. There are two different approaches that have been followed to produce fluorinated graphene: (1) fluorination of pre-synthesized graphene: This approach entails treating graphene prepared by mechanical exfoliation or by CVD growth with fluorinating agent such as XeF2, or F-based plasmas; (2) Exfoliation of multilayered graphite fluorides: Both mechanical exfoliation and liquid phase exfoliation of graphite fluoride can be readily accomplished [F. Karlicky, et al. “Halogenated Graphenes: Rapidly Growing Family of Graphene Derivatives” ACS Nano, 2013, 7 (8), pp 6434-6464].
Interaction of F2 with graphite at high temperature leads to covalent graphite fluorides (CF)n or (C2F)n, while at low temperatures graphite intercalation compounds (GIC) CxF (2≤x≤24) form. In (CF)n carbon atoms are sp3-hybridized and thus the fluorocarbon layers are corrugated consisting of trans-linked cyclohexane chairs. In (C2F)n only half of the C atoms are fluorinated and every pair of the adjacent carbon sheets are linked together by covalent C—C bonds. Systematic studies on the fluorination reaction showed that the resulting F/C ratio is largely dependent on the fluorination temperature, the partial pressure of the fluorine in the fluorinating gas, and physical characteristics of the graphite precursor, including the degree of graphitization, particle size, and specific surface area. In addition to fluorine (F2), other fluorinating agents may be used, although most of the available literature involves fluorination with F2 gas, sometimes in presence of fluorides.
For exfoliating a layered precursor material to the state of individual single graphene layers or few-layers, it is necessary to overcome the attractive forces between adjacent layers and to further stabilize the layers. This may be achieved by either covalent modification of the graphene surface by functional groups or by non-covalent modification using specific solvents, surfactants, polymers, or donor-acceptor aromatic molecules. The process of liquid phase exfoliation includes ultra-sonic treatment of a graphite fluoride in a liquid medium to produce graphene fluoride sheets dispersed in the liquid medium. The resulting dispersion can be directly used in the graphene deposition of polymer component surfaces.
The nitrogenation of graphene can be conducted by exposing a graphene material, such as graphene oxide, to ammonia at high temperatures (200-400° C.). Nitrogenated graphene could also be formed at lower temperatures by a hydrothermal method; e.g. by sealing GO and ammonia in an autoclave and then increased the temperature to 150-250° C. Other methods to synthesize nitrogen doped graphene include nitrogen plasma treatment on graphene, arc-discharge between graphite electrodes in the presence of ammonia, ammonolysis of graphene oxide under CVD conditions, and hydrothermal treatment of graphene oxide and urea at different temperatures.
For the purpose of defining the claims of the instant application, NGPs or graphene materials include discrete sheets/platelets of single-layer and multi-layer (typically less than 10 layers, the few-layer graphene) pristine graphene, graphene oxide, reduced graphene oxide (RGO), graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, doped graphene (e.g. doped by B or N). Pristine graphene has essentially 0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight. Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen. Other than pristine graphene, all the graphene materials have 0.001%-50% by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.). These materials are herein referred to as non-pristine graphene materials. The presently invented graphene can contain pristine or non-pristine graphene and the invented method allows for this flexibility. These graphene sheets all can be chemically functionalized.
As illustrated in
Step (A) of combining metal with graphene sheets may include a procedure of depositing particles or coating of the lithium-attracting metal or sodium-attracting metal onto surfaces of the multiple graphene sheets to obtain the graphene/metal mixture, which comprises multiple metal-deposited graphene sheets. Decoration or deposition of a Li- or Na-attracting metal onto surfaces of graphene sheets prior to being subjected to graphene ball formation may be accomplished via using various depositing or coating means (e.g. melt dipping, solution deposition, chemical vapor deposition, physical vapor deposition, sputtering, electrochemical deposition, spray coating, plasma coating, or a combination thereof).
For the purpose of defining the scope of the claims, the lithium- or sodium-attracting metal recited in Step (A) may include a precursor to this metal; such a precursor may be later chemically or thermally converted to the desired metal. For instance, graphene surfaces may be coated with HAuCl4 or silver nitrate which is then thermally converted to Au or Ag when the graphene balls are heated. Another example is to deposit zinc chloride on graphene surfaces (e.g. via salt solution dipping and drying) and use hydrogen and methane to chemically convert this precursor to Zn metal at a later stage (e.g. before or after graphene deposition). There are many metal precursors or metal salts (e.g. metal acetate, metal nitrate, metal sulfate, metal phosphate, metal halogenated, etc.) to metals that are well-known in the art.
In some embodiments, the process may include (a) depositing a metal-containing precursor (e.g. an organo-metallic molecule or a metal salt) onto the surfaces of multiple graphene sheets to form precursor-coated graphene sheets or mixing a metal-containing precursor with multiple graphene sheets to form metal precursor/graphene mixture; (b) forming the precursor-coated graphene sheets or metal precursor/graphene mixture into graphene balls containing metal precursor therein; and (c) heat treating the graphene balls to thermally convert or chemically treating the graphene balls to chemically reduce the metal precursor to a metal phase, wherein the metal resides in the pores of the resulting particulates or adheres to graphene sheet surfaces in the particulates.
Step (B) of forming graphene balls may be conducted via several procedures. Two examples are given below: (i) Ball milling of a mixture containing multiple graphene sheets and particles of the metal (or solution of metal precursor; e.g. silver nitrate dissolved in water or alcohol): These graphene sheets can contain pristine graphene, graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, or a combination thereof. (ii) Spray-drying of a suspension containing multiple graphene sheets and metal particles (or precursor to metal) dispersed in a liquid medium (e.g. water or organic solvent).
Such graphene balls (containing an ion-attracting metal) or graphene-metal particulates may be formed (e.g. along with a binder) into a shape and dimensions of a desired electrode (an anode). Such an electrode can be pre-lithiated or attached to a lithium foil and then directly impregnated with an electrolyte to form an electrolyte-impregnated electrode layer (e.g. anode). The anode layer, a separator, and a cathode layer can then be laminated (with or without an anode current collector and/or cathode current collector) to form a lithium battery cell, which is then packaged in an envelop or casing (e.g. laminated plastic-aluminum housing). Alternatively, an un-impregnated anode layer, a separator layer, and an un-impregnated cathode layer are laminated together (with or without externally added current collectors) to form a battery cell, which is then inserted in a housing and impregnated with an electrolyte to form a packaged lithium battery cell. A sodium cell may be produced in a similar manner.
In the above processes, the ball milling procedure is preferably conducted by using an energy impacting apparatus selected from a double cone mixer, double cone blender, vibratory ball mill, planetary ball mill, high energy mill, basket mill, agitator ball mill, cryogenic ball mill, micro ball mill, tumbler ball mill, attritor, continuous ball mill, stirred ball mill, pressurized ball mill, plasma-assisted ball mill, freezer mill, vibratory sieve, bead mill, nano bead mill, ultrasonic homogenizer mill, centrifugal planetary mixer, vacuum ball mill, or resonant acoustic mixer. In certain preferred embodiments, the procedure of operating the energy impacting apparatus is conducted in a continuous manner using a continuous energy impacting device. The milling media may be selected from particles of a metal or metal alloy, a glass, a ceramic, a polymer, or a combination thereof.
There are three broad categories of methods that can be implemented to produce graphene/metal particulates. These include physical methods, physico-chemical methods, and chemical methods. The physical methods include pan-coating, air-suspension coating, centrifugal extrusion, vibration nozzle coating, and spray-drying methods. The physico-chemical methods include ionotropic gelation and coacervation-phase separation methods. The chemical methods include interfacial polycondensation, interfacial cross-linking, in-situ polymerization, and matrix polymerization. Several preferred processes are briefly discussed below:
Pan-coating method: The pan coating process involves tumbling a mixture of graphene sheets, particles of a Li or Na ion-attracting metal, an optional adhesive, and an optional conductive additive in a pan or a similar device while the encapsulating material (e.g. graphene sheets dispersed in a monomer/oligomer, polymer melt, polymer/solvent solution) is applied slowly until a desired encapsulating shell thickness is attained.
Air-suspension coating method: In the air suspension coating process, a mixture of graphene sheets, particles of a Li or Na ion-attracting metal, an optional adhesive, and an optional conductive additive is dispersed into the supporting air stream in an encapsulating chamber. A controlled stream of a suspension comprising graphene sheets dispersed in a polymer-solvent solution (e.g. polymer or its monomer or oligomer dissolved in a solvent; or its monomer or oligomer alone in a liquid state) is concurrently introduced into this chamber, allowing the solution to hit and coat the suspended mixture particles. These suspended particles are coated with polymer/graphene sheets while the volatile solvent is removed, producing balls of polymer-bonded graphene sheets along with the metal particles supported thereon.
Vibrational nozzle encapsulation method: Graphene balls containing graphene sheets and metal particles (or metal-coated graphene sheets) can be conducted using a laminar flow through a nozzle and vibration of the nozzle or the liquid. The vibration has to be done in resonance with the Rayleigh instability, leading to very uniform droplets. The liquid can consist of any liquids with limited viscosities (1-50,000 mPa·s): emulsions, suspensions or slurry containing the metal particles and graphene sheets dispersed in a liquid medium.
Spray-drying: Spray drying may be used to combine graphene sheets and metal particles (or metal-decorated graphene sheets) into graphene balls from a suspension comprising multiple graphene sheets and metal particles (or metal-decorated graphene sheets) suspended in a liquid medium or a polymer solution. In spray drying, the liquid feed (solution or suspension) is atomized to form droplets which, upon contacts with hot gas, allow solvent to get vaporized and graphene sheets and metal particles (or metal-coated graphene sheet) naturally self-assemble into graphene balls.
The process may further comprise a step of impregnating lithium metal or sodium metal into the graphene particulates, wherein the lithium metal or sodium metal is in physical contact with the lithium-attracting metal or sodium-attracting metal to form lithium-preloaded or sodium-preloaded graphene particulates.
The process may further comprise a step of adding 0.01% to 40% by weight of a binder, adhesive, or matrix material to help hold the multiple graphene sheets in the graphene ball together as a composite ball. This may be accomplished for example by including the adhesive/binder/matrix material in the suspension prior to the graphene ball forming procedure, or by spraying a binder or matrix material onto the surfaces of graphene particulates after formation. The binder, adhesive, or matrix material may comprise an electron-conducting or lithium ion-conducting material. The electron-conducting material may be selected from an intrinsically conducting polymer, a pitch, a metal, a combination thereof, or a combination thereof with carbon, wherein this metal does not include Au, Ag, Mg, Zn, Ti, Li, Na, K, Al, Fe, Mn, Co, Ni, Sn, V, Cr, or an alloy thereof. The intrinsically conducting polymer is selected from polyaniline, polypyrrole, polythiophene, polyfuran, polyacetylene, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof.
The graphene balls may comprise therein a lithium ion-or sodium ion-conducting material selected from Li2CO3, Li2O, Li2C2O4, LiOH, LiX, ROCO2Li, HCOLi, ROLi, (ROCO2Li)2, (CH2OCO2Li)2, Li2S, LixSOy, or a combination thereof, wherein X=F, Cl, I, or Br, R=a hydrocarbon group, 0<x≤1, 1≤y≤4.
The lithium ion-conducting material may contain a lithium salt selected from lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium borofluoride (LiBF4), lithium hexafluoroarsenide (LiAsF6), lithium trifluoro-methanesulfonate (LiCF3SO3), bis-trifluoromethyl sulfonylimide lithium (LiN(CF3SO2)2), lithium bis(oxalato)borate (LiBOB), lithium oxalyldifluoroborate (LiBF2C2O4), lithium nitrate (LiNO3), Li-fluoroalkyl-phosphate (LiPF3(CF2CF3)3), lithium bisperfluoro-ethylsulfonylimide (LiBETI), lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonimide (LiTFSI), an ionic liquid-based lithium salt, or a combination thereof. These salts can also be used as a lithium salt of an electrolyte for a lithium battery.
Alternatively or additionally, the lithium ion-conducting material may comprise a lithium ion-conducting polymer selected from poly(ethylene oxide) (PEO), Polypropylene oxide (PPO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), poly(vinylidene fluoride) (PVdF), Poly bis-methoxy ethoxyethoxide-phosphazenex, Polyvinyl chloride, Polydimethylsiloxane, poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP), a sulfonated derivative thereof, or a combination thereof. These materials may be added into the suspension prior to graphene ball formation.
In certain embodiments, the graphene balls comprise therein an electron-conducting material selected from an expanded graphite flake, carbon nanotube, carbon nano-fiber, carbon fiber, carbon particle, graphite particle, carbon black, acetylene black, pitch, an electron-conducting polymer, or a combination thereof. The electron-conducting polymer is preferably selected from polyaniline, polypyrrole, polythiophene, polyfuran, polyacetylene, a bi-cyclic polymer, a sulfonated derivative thereof, or a combination thereof. These materials may be added into the suspension prior to graphene ball formation.
The process may further comprise a step of combining a plurality of presently disclosed graphene particulates together to form an anode electrode. The process may further comprise a step of combining a cathode, the disclosed anode electrode, an optional lithium source or a sodium source in ionic contact with said anode electrode, and an electrolyte in ionic contact with both the cathode and the anode electrode to form an alkali metal battery cell. The lithium source is selected from foil, particles, or filaments of lithium metal or lithium alloy having no less than 80% by weight of lithium element in the lithium alloy; or wherein the sodium source is selected from foil, particles, or filaments of sodium metal or sodium alloy having no less than 80% by weight of sodium element in the sodium alloy. The lithium ion or sodium ion source may not be required if the cathode active material has some built-in lithium or sodium atoms (e.g. lithium transition metal oxide, NMC, NCA, etc.) that can be released during the battery charge procedure.
The graphene-metal particulates contain single-layer or few-layer graphene sheets in the encapsulating shell, wherein the few-layer graphene sheets have 2-10 layers of stacked graphene planes having an inter-plane spacing d002 from 0.3354 nm to 0.36 nm as measured by X-ray diffraction and the single-layer or few-layer graphene sheets contain a pristine graphene material having essentially zero % of non-carbon elements, or a non-pristine graphene material having 0.01% to 25% by weight of non-carbon elements (more typically <15%) wherein the non-pristine graphene is selected from graphene oxide, reduced graphene oxide, graphene fluoride, graphene chloride, graphene bromide, graphene iodide, hydrogenated graphene, nitrogenated graphene, chemically functionalized graphene, or a combination thereof.
The graphene particulates, without the Li- or Na-attracting metal, typically have a density from 0.001 to 1.7 g/cm3, a specific surface area from 50 to 2,630 m2/g. In a preferred embodiment, the graphene sheets contain stacked graphene planes having an inter-planar spacing d002 from 0.3354 nm to 0.40 nm as measured by X-ray diffraction.
The gaps between the free ends of the graphene sheets may be advantageously bonded by an intrinsically conducting polymer, a pitch, a metal, etc. Due to these unique chemical composition (including oxygen or fluorine content, etc.), morphology, crystal structure (including inter-graphene spacing), and structural features (e.g. degree of orientations, few defects, chemical bonding and no gap between graphene sheets, and substantially no interruptions along graphene plane directions), the graphene particulates have a unique combination of outstanding thermal conductivity, electrical conductivity, mechanical strength, and elasticity.
The aforementioned features and characteristics make the graphene-metal hybrid particulates an ideal battery anode active material or a lithiating agent for the following reasons.
Electrolyte is an important ingredient in a battery. A wide range of electrolytes can be used for practicing the instant disclosure. Most preferred are non-aqueous liquid, polymer gel, and solid-state electrolytes although other types can be used. Polymer, polymer gel, and solid-state electrolytes are preferred over liquid electrolyte.
The non-aqueous electrolyte to be employed herein may be produced by dissolving an electrolytic salt in a non-aqueous solvent. Any known non-aqueous solvent which has been employed as a solvent for a lithium secondary battery can be employed. A non-aqueous solvent mainly consisting of a mixed solvent comprising ethylene carbonate (EC) and at least one kind of non-aqueous solvent whose melting point is lower than that of aforementioned ethylene carbonate and whose donor number is 18 or less (hereinafter referred to as a second solvent) may be preferably employed. This non-aqueous solvent is advantageous in that it is (a) effective in suppressing the reductive or oxidative decomposition of electrolyte; and (b) high in conductivity. A non-aqueous electrolyte solely composed of ethylene carbonate (EC) is advantageous in that it is relatively stable against carbonaceous filament materials. However, the melting point of EC is relatively high, 39 to 40° C., and the viscosity thereof is relatively high, so that the conductivity thereof is low, thus making EC alone unsuited for use as a secondary battery electrolyte to be operated at room temperature or lower. The second solvent to be used in a mixture with EC functions to make the viscosity of the solvent mixture lower than that of EC alone, thereby promoting the ion conductivity of the mixed solvent. Furthermore, when the second solvent having a donor number of 18 or less (the donor number of ethylene carbonate is 16.4) is employed, the aforementioned ethylene carbonate can be easily and selectively solvated with lithium ion, so that the reduction reaction of the second solvent with the carbonaceous material well developed in graphitization is assumed to be suppressed. Further, when the donor number of the second solvent is controlled to not more than 18, the oxidative decomposition potential to the lithium electrode can be easily increased to 4 V or more, so that it is possible to manufacture a lithium secondary battery of high voltage.
Preferable second solvents are dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), ethyl propionate, methyl propionate, propylene carbonate (PC), gamma.-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), toluene, xylene and methyl acetate (MA). These second solvents may be employed singly or in a combination of two or more. More desirably, this second solvent should be selected from those having a donor number of 16.5 or less. The viscosity of this second solvent should preferably be 28 cps or less at 25° C.
The electrolytic salts to be incorporated into a non-aqueous electrolyte may be selected from a lithium salt such as lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), lithium borofluoride (LiBF4), lithium hexafluoroarsenide (LiAsF6), lithium trifluoro-methanesulfonate (LiCF3SO3) and bis-trifluoromethyl sulfonylimide lithium [LiN(CF3SO2)2]. Among them, LiPF6, LiBF4 and LiN(CF3SO2)2 are preferred. The content of aforementioned electrolytic salts in the non-aqueous solvent is preferably 0.5 to 3.5 mol/l.
For sodium metal batteries, the organic electrolyte may contain an alkali metal salt preferably selected from sodium perchlorate (NaClO4), potassium perchlorate (KClO4), sodium hexafluorophosphate (NaPF6), potassium hexafluorophosphate (KPF6), sodium borofluoride (NaBF4), potassium borofluoride (KBF4), sodium hexafluoroarsenide, potassium hexafluoroarsenide, sodium trifluoro-methanesulfonate (NaCF3SO3), potassium trifluoro-methanesulfonate (KCF3SO3), bis-trifluoromethyl sulfonylimide sodium (NaN(CF3SO2)2), bis-trifluoromethyl sulfonylimide potassium (KN(CF3SO2)2), an ionic liquid salt, or a combination thereof.
The ionic liquid is composed of ions only. Ionic liquids are low melting temperature salts that are in a molten or liquid state when above a desired temperature. For instance, a salt is considered as an ionic liquid if its melting point is below 100° C. If the melting temperature is equal to or lower than room temperature (25° C.), the salt is referred to as a room temperature ionic liquid (RTIL). The IL salts are characterized by weak interactions, due to the combination of a large cation and a charge-delocalized anion. This results in a low tendency to crystallize due to flexibility (anion) and asymmetry (cation).
A typical and well-known ionic liquid is formed by the combination of a 1-ethyl-3-methylimidazolium (EMI) cation and an N,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combination gives a fluid with an ionic conductivity comparable to many organic electrolyte solutions and a low decomposition propensity and low vapor pressure up to ˜300-400° C. This implies a generally low volatility and non-flammability and, hence, a much safer electrolyte for batteries.
Ionic liquids are basically composed of organic ions that come in an essentially unlimited number of structural variations owing to the preparation ease of a large variety of their components. Thus, various kinds of salts can be used to design the ionic liquid that has the desired properties for a given application. These include, among others, imidazolium, pyrrolidinium and quaternary ammonium salts as cations and bis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide, and hexafluorophosphate as anions. Based on their compositions, ionic liquids come in different classes that basically include aprotic, protic and zwitterionic types, each one suitable for a specific application.
Common cations of room temperature ionic liquids (RTILs) include, but not limited to, tetraalkylammonium, di-, tri-, and tetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium, dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium. Common anions of RTILs include, but not limited to, BF4−, B(CN)4−, CH3BF3−, CH2CHBF3−, CF3BF3−, C2F5BF3−, n-C3F7BF3−, n-C4F9BF3−, PF6−, CF3CO2−, CF3SO3−, N(SO2CF3)2−, N(COCF3)(SO2CF3)−, N(SO2F)2−, N(CN)2−, C(CN)3−, SCN−, SeCN−, CuCl2−, AlCl4−, F(HF)2.3−, etc. Relatively speaking, the combination of imidazolium- or sulfonium-based cations and complex halide anions such as AlCl4−, BF4−, CF3CO2−, CF3SO3−, NTf2−, N(SO2F)2−, or F(HF)2.3− results in RTILs with good working conductivities.
RTILs can possess archetypical properties such as high intrinsic ionic conductivity, high thermal stability, low volatility, low (practically zero) vapor pressure, non-flammability, the ability to remain liquid at a wide range of temperatures above and below room temperature, high polarity, high viscosity, and wide electrochemical windows. These properties, except for the high viscosity, are desirable attributes when it comes to using an RTIL as an electrolyte ingredient (a salt and/or a solvent) in a battery.
The cathode active material may be selected from a wide variety of oxides, such as lithium-containing nickel oxide, cobalt oxide, nickel-cobalt oxide, vanadium oxide, and lithium iron phosphate. These oxides may contain a dopant, which is typically a metal element or several metal elements. The cathode active material may also be selected from chalcogen compounds, such as titanium disulfate, molybdenum disulfate, and metal sulfides. More preferred are lithium cobalt oxide (e.g., LixCoO2 where 0.8≤x≤1), lithium nickel oxide (e.g., LiNiO2), lithium manganese oxide (e.g., LiMn2O4 and LiMnO2), lithium transition metal oxides (e.g. NCM, NCA, etc.), lithium iron phosphate, lithium manganese-iron phosphate, lithium vanadium phosphate, and the like. Sulfur or lithium polysulfide may also be used in a Li—S cell.
The rechargeable lithium metal batteries can make use of non-lithiated compounds, such as TiS2, MoS2, MnO2, CoO2, V3O8, and V2O5, as the cathode active materials. The lithium vanadium oxide may be selected from the group consisting of VO2, LixVO2, V2O5, LixV2O5, V3O8, LixV3O8, LixV3O7, V4O9, LixV4O9, V6O13, LixV6O13, their doped versions, their derivatives, and combinations thereof, wherein 0.1<x<5. In general, the inorganic material-based cathode materials may be selected from a metal carbide, metal nitride, metal boride, metal dichalcogenide, or a combination thereof. Preferably, the desired metal oxide or inorganic material is selected from an oxide, dichalcogenide, trichalcogenide, sulfide, selenide, or telluride of niobium, zirconium, molybdenum, hafnium, tantalum, tungsten, titanium, vanadium, chromium, cobalt, manganese, iron, or nickel in a nanowire, nano-disc, nano-ribbon, or nano platelet form. These materials can be in the form of a simple mixture with sheets of a graphene material, but preferably in a nano particle or nano coating form that that is physically or chemically bonded to a surface of the graphene sheets.
Preferably, the cathode active material for a sodium metal battery contains a sodium intercalation compound or a potassium intercalation compound selected from NaFePO4, KFePO4, Na(1-x)KxPO4, Na0.7FePO4, Na1.5VOPO4F0.5, Na3V2(PO4)3, Na3V2(PO4)2F3, Na2FePO4F , NaFeF3, NaVPO4F, KVPO4F, Na3V2(PO4)2F3, Na1.5VOPO4F0.5, Na3V2(PO4)3, NaV6O15, NaxVO2, Na0.33V2O5, NaxCoO2, Na2/3[Ni1/3Mn2/3]O2, Nax(Fe1/2Mn1/2)O2, NaxMnO2, NaxK(1-x)MnO2, Na0.44MnO2, Na0.44MnO2/C, Na4Mn9O18, NaFe2Mn(PO4)3, Na2Ti3O7, Ni1/3Mn1/3Co1/3O2, Cu0.56Ni0.44HCF, NiHCF, NaxMnO2, NaCrO2, KCrO2, Na3Ti2(PO4)3, NiCo2O4, Ni3S2/FeS2, Sb2O4, Na4Fe(CN)6/C, NaV1-xCrxPO4F, SezSy (y/z=0.01 to 100), Se, Alluaudites, or a combination thereof, wherein x is from 0.1 to 1.0.
The organic material or polymeric material-based cathode materials may be selected from Poly(anthraquinonyl sulfide) (PAQS), a lithium oxocarbon, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), poly(anthraquinonyl sulfide), pyrene-4,5,9,10-tetraone (PYT), polymer-bound PYT, Quino(triazene), redox-active organic material, Tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), 2,3,6,7,10,11-hexamethoxytriphenylene (HMTP), poly(5-amino-1,4-dyhydroxy anthraquinone) (PADAQ), phosphazene disulfide polymer ([(NPS2)3]n), lithiated 1,4,5,8-naphthalenetetraol formaldehyde polymer, Hexaazatrinaphtylene (HATN), Hexaazatriphenylene hexacarbonitrile (HAT(CN)6), 5-B enzylidene hydantoin, Isatine lithium salt, Pyromellitic diimide lithium salt, tetrahydroxy-p-benzoquinone derivatives (THQLi4), N,N′-diphenyl-2,3,5,6-tetraketopiperazine (PHP), N,N′-diallyl-2,3,5,6-tetraketopiperazine (AP), N,N′-dipropyl-2,3,5,6-tetraketopiperazine (PRP), a thioether polymer, a quinone compound, 1,4-benzoquinone, 5,7,12,14-pentacenetetrone (PT), 5-amino-2,3-dihydro-1,4-dyhydroxy anthraquinone (ADDAQ), 5-amino-1,4-dyhydroxy anthraquinone (ADAM), calixquinone, Li4C6O6, Li2C6O6, Li6C6O6, NaxC6O6 (x=1-3), Na2(C6H2O4), Na2C8H4O4(Na terephthalate), Na2C6H4O4(Na trans-trans-muconate), or a combination thereof.
The thioether polymer is selected from Poly[methanetetryl-tetra(thiomethylene)] (PMTTM), Poly(2,4-dithiopentanylene) (PDTP), a polymer containing Poly(ethene-1,1,2,2-tetrathiol) (PETT) as a main-chain thioether polymers, a side-chain thioether polymer having a main-chain consisting of conjugating aromatic moieties, and having a thioether side chain as a pendant, Poly(2-phenyl-1,3-dithiolane) (PPDT), Poly(1,4-di(1,3-dithiolan-2-yl)benzene) (PDDTB), poly(tetrahydrobenzodithiophene) (PTHBDT), poly[1,2,4,5-tetrakis(propylthio) benzene] (PTKPTB, or poly[3,4(ethylenedithio)thiophene] (PEDTT).
The organic material that can be used as a cathode active material in a lithium metal battery or sodium metal battery may include a phthalocyanine compound selected from copper phthalocyanine, zinc phthalocyanine, tin phthalocyanine, iron phthalocyanine, lead phthalocyanine, nickel phthalocyanine, vanadyl phthalocyanine, fluorochromium phthalocyanine, magnesium phthalocyanine, manganous phthalocyanine, dilithium phthalocyanine, aluminum phthalocyanine chloride, cadmium phthalocyanine, chlorogallium phthalocyanine, cobalt phthalocyanine, silver phthalocyanine, a metal-free phthalocyanine, a chemical derivative thereof, or a combination thereof.
The following examples are used to illustrate some specific details about the best modes of practicing the instant disclosure and should not be construed as limiting the scope of the disclosure.
Graphite oxide was prepared by oxidation of graphite flakes with an oxidizer liquid consisting of sulfuric acid, sodium nitrate, and potassium permanganate at a ratio of 4:1:0.05 at 30° C. When natural graphite flakes (particle sizes of 14 μm) were immersed and dispersed in the oxidizer mixture liquid for 48 hours, the suspension or slurry appears and remains optically opaque and dark. After 48 hours, the reacting mass was rinsed with water 3 times to adjust the pH value to at least 3.0. A final amount of water was then added to prepare a series of GO-water suspensions.
For incorporation of higher melting point metals (e.g. Au, Ag, Ni, Co, Mn, Fe, and Ti) as a lithium- or sodium-attracting metal in graphene particulates, a small but controlled amount of a precursor material (e.g. HAuCl4, silver nitrate, or nickel acetate) was separately added into separate samples of Go-water suspension. The resulting slurries were then spray-dried into graphene particulates, wherein the graphene sheets are coated with a metal precursor. Upon heating of the graphene particulates at a desired temperature (typically 450-750° C.) for a desired length of time (typically 0.5-2.0 hours), the precursor became nano-scaled particles of metal (e.g. Au, Ag, and Ni metal) bonded on graphene sheet surfaces.
In order to determine the relative stability of the metal-containing graphene ball-based anode structure, the voltage profiles of symmetric layered Li-metal-decorated graphene particulate-containing layer electrode cells, symmetric layered Li-(metal free) graphene particulate electrode cells, and the bare Li foil counterparts were obtained through over 200 cycles at nominal current density of 1 mA/cm2. The graphene particulate-containing layer electrode was made by the conventional slurry coating procedure using PVDF as a binder.
The symmetric layered Li-graphene particulate electrode cells exhibited stable voltage profiles with negligible hysteresis, whereas the bare Li foils displayed a rapid increase in hysteresis during cycling, by 90% after 100 cycles. The hysteresis growth rate of the symmetric layered Li-(metal free) graphene electrode cell is significantly greater than that of the symmetric layered Li-metal-decorated graphene particulate-containing layer electrode cell, but lower than that of the bare Li foil cell. For symmetric layered Li-metal-decorated graphene particulate-containing layer electrode cells, flat voltage plateau at both the charging and discharging states can be retained throughout the whole cycle without obvious increases in hysteresis. This is a significant improvement compared with bare Li electrodes, which showed fluctuating voltage profiles with consistently higher overpotential at both the initial and final stages of each stripping/plating process. After 320 cycles, there is no sign of dendrite formation and the lithium deposition is very even in symmetric layered Li-metal-decorated graphene particulate-containing layer electrode cells. For the symmetric layered Li-(metal-free) metal-decorated graphene particulate-containing layer electrode cells, some lithium tends to deposit unevenly on external surfaces of pores, instead of fully entering the pores. Typically, for bare Li foil electrodes, dendrite begins to develop in less than 30 cycles.
Meso-carbon microbeads (MCMBs) were supplied from China Steel Chemical Co., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm3 with a median particle size of about 16 μm. MCMB (10 grams) were intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated MCMBs were repeatedly washed in a 5% solution of HCl to remove most of the sulfate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was no less than 4.5. The slurry was then subjected ultrasonication for 10-100 minutes to produce GO suspensions. TEM and atomic force microscopic studies indicate that most of the GO sheets were single-layer graphene when the oxidation treatment exceeded 72 hours, and 2- or 3-layer graphene when the oxidation time was from 48 to 72 hours. The GO sheets contain oxygen proportion of approximately 35%-47% by weight for oxidation treatment periods of 48-96 hours. Silver nanowires (AgNW) and fine Cu particles were separately added into two GO-water suspension samples, which were then spray-dried to produce graphene balls containing AgNWs and Cu particles therein, respectively.
Recognizing the possibility of the high defect population in GO sheets acting to reduce the conductivity of individual graphene plane, we decided to study if the use of pristine graphene sheets (non-oxidized and oxygen-free, non-halogenated and halogen-free, etc.) can lead to graphene balls having a higher thermal or electrical conductivity. Pristine graphene sheets were produced by using the direct ultrasonication or liquid-phase production process.
In a typical procedure, five grams of graphite flakes, ground to approximately 20 μm or less in sizes, were dispersed in 1,000 mL of deionized water (containing 0.1% by weight of a dispersing agent, Zonyl® FSO from DuPont) to obtain a suspension. An ultrasonic energy level of 85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation, and size reduction of graphene sheets for a period of 15 minutes to 2 hours. The resulting graphene sheets are pristine graphene that have never been oxidized and are oxygen-free and relatively defect-free. There are substantially no other non-carbon elements.
Nickel nitrate hexahydrant, Ni(NO3)2.6H2O, 0.1 M 250 μL and a similar copper nitrate solution were then added to a suspension containing 150 mg graphene dispersed in water and the mixture was sonicated for 1 hour. The suspension was divided up into two samples. One sample was subjected to spray-drying to produce graphene balls containing metal precursor ingredients therein (the first powder mass). The other sample was left to dry at 60° C. in a vacuum oven to obtain the second powder mass. Both powder mass samples, each containing graphene-nickel nitrate/copper nitrate mixture, were then heated to 700° C. under Argon atmosphere for 1 hour inside a tube furnace. Each resulting powder product contains a well-blended mixture of Cu and Ni nano particles deposited on graphene surfaces. The product from the first powder mass was already in the form of desired metal-containing graphene balls and was ready to be made into an anode electrode for a lithium metal battery or sodium metal battery.
The product from the second powder mass was further ground into fine powder particles and then made into metal-containing, conducting polymer bonded graphene balls through a pan coating procedure using a solution of PEDOT/PSS dissolved in water. It may be noted that Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is a polymer mixture of two ionomers. One component is made up of sodium polystyrene sulfonate, which is a sulfonated polystyrene. Part of the sulfonyl groups are deprotonated and carry a negative charge. The other component poly(3,4-ethylenedioxythiophene) or PEDOT is a conjugated polymer, polythiophene, which carries positive charges. Together the two charged polymers form a macromolecular salt, which is soluble in water.
The two types of metal-containing graphene balls were incorporated as an anode active material in sodium-ion batteries. Electrochemical characterization was conducted by using CR2032-type coin cell wherein Na metal was used as the counter and reference electrodes. To make slurry, active material (80 wt %), Super P (10 wt %) and PAA binder (10 wt %) were mixed in mortal and then N-methyl-2-pyrrolidone (NMP) was added to regulate the viscosity of slurry. The slurry was casted on Cu foil and dried in a vacuum oven at 150° C. for 10 h. Disc-shape electrodes were punched into 12 mm size. The average loading mass of electrodes was 1.1 mg/cm2. Also, 1 M solution of NaPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1, v/v) with 5% flouro-ethylene carbonate (FEC) was employed as an electrolyte, and glass fiber fabric was used as a porous separator. The coin cell was fabricated in an Ar-filled glove box. Galvanostatic charge-discharge cycling test was performed between 0.01 and 2 V vs. Na+/Na at various rates or current densities (0.1 to 2 A/g). We have observed that the conductive polymer-based adhesive appears to improve the cycling stability of the metal-containing graphene balls.
Several processes have been used by us to produce GF, but only one process is herein described as an example. In a typical procedure, highly exfoliated graphite (HEG) was prepared from intercalated compound C2F.xClF3. HEG was further fluorinated by vapors of chlorine trifluoride to yield fluorinated highly exfoliated graphite (FHEG). Pre-cooled Teflon reactor was filled with 20-30 mL of liquid pre-cooled ClF3, the reactor was closed and cooled to liquid nitrogen temperature. Then, no more than 1 g of HEG was put in a container with holes for ClF3 gas to access and situated inside the reactor. In 7-10 days a gray-beige product with approximate formula C2F was formed.
Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixed with 20-30 mL of an organic solvent (methanol, but ethanol, 1-propanol, 2-propanol, 1-butanol, tert-butanol, isoamyl alcohol all can be used) and subjected to an ultrasound treatment (280 W) for 30 min, leading to the formation of homogeneous yellowish dispersion. Silver nano-wires were then dispersed into the dispersion and spray-dried into graphene balls containing Ag nano-wires supported on graphene fluoride sheets.
Graphene oxide (GO), synthesized in Example 1, was finely ground with different proportions of urea and the pelletized mixture heated in a microwave reactor (900 W) for 30 s. The product was washed several times with deionized water and vacuum dried. In this method graphene oxide gets simultaneously reduced and doped with nitrogen. The products obtained have nitrogen contents of 14.7, 18.2 and 17.5 wt % respectively as measured by elemental analysis. These nitrogenated graphene sheets remain dispersible in water. The resulting suspensions were then added with particles of Zn and Cu, respectively and then spray-dried into graphene balls.
In a conventional cell, an electrode (e.g. cathode) is typically composed of 85% an electrode active material (e.g. LixV2O5, NCM, NCA, sodium polysulfide, lithium polysulfide, etc.), 5% Super-P (acetylene black-based conductive additive), and 10% PTFE, which were mixed in NMP solvent to form a slurry. The slurry was then coated on Al foil. The thickness of electrode was around 50-150 μm. A wide variety of cathode active materials were implemented to produce lithium metal batteries and sodium metal batteries. Anode layers were similarly made using metal-containing graphene balls as an anode active material. Some of the graphene balls were pre-loaded with lithium or sodium metal. Several lithium-ion cells were also made that comprised lithium-preloaded graphene particulates as a first anode active material and a conventional anode material (e.g. particles of graphite or Si) as a second anode active material in the anode. The graphene particles pre-loaded with lithium were used as a pre-lithiating agent for the conventional anode active material.
For each sample, both coin-size and pouch cells were assembled in a glove box. The charge storage capacity was measured with galvanostatic experiments using an Arbin SCTS electrochemical testing instrument. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were conducted on an electrochemical workstation (CHI 660 System, USA).
For each sample, several current density (representing charge/discharge rates) were imposed to determine the electrochemical responses, allowing for calculations of energy density and power density values required of the construction of a Ragone plot (power density vs. energy density).
The data on the gravimetric power density vs. energy density of two sets of lithium metal cells were obtained: (a) first cell containing a layer of Zn-containing nitrogenated graphene balls bonded by PVDF, in physical contact with a lithium foil, as the anode active material; (b) the second cell containing no lithium-attracting metal (Zn) inside the graphene balls. These data indicate that the energy density and power density ranges of these two cells are comparable. However, SEM examination of the cell samples, taken after 30 charge-discharge cycles, indicates that the sample containing a Li-attracting metal has essentially all the lithium ions returning from the cathode during charge being encased inside the graphene balls, having no tendency to form lithium dendrites. In contrast, for the cell containing no lithium-attracting metal inside the graphene balls, lithium metal tends to get re-plated on external surfaces of graphene particulates in a less uniform manner. Further surprisingly, as shown in
Shown in
In conclusion, we have successfully developed a new, novel, unexpected, and patently distinct class of metal-containing graphene balls or particulates that can be used in a lithium metal battery or sodium metal battery for overcoming the dendrite issues. This class of new materials has now made it possible to use lithium metal and sodium metal batteries that have much higher energy densities as compared to the conventional lithium-ion cells. Additionally, the graphene particulates, preloaded with lithium or sodium, may be used as a pre-lithiating agent or pre-sodiating agent for a conventional lithium-ion battery or sodium-ion battery, respectively.
