This invention relates generally to the field of electrochemical energy storage devices and, more particularly, to a totally new hybrid electrode (the electrode itself being a hybrid) and a super-hybrid cell that contains this hybrid electrode. The intercalation-free active material of this hybrid electrode enables a charge/discharge behavior characteristic of a surface-mediated cell (SMC). The super-hybrid cell operates primarily on the exchange of lithium ions between anode surfaces and cathode surfaces, plus some amount of lithium being exchanged between interior of an electrode and surfaces/interior of an opposing electrode.
Supercapacitors are being considered for electric vehicle (EV), renewable energy storage, and modern grid applications. The high volumetric capacitance density of a supercapacitor derives from using porous electrodes to create a large surface area conducive to the formation of diffuse electric double layer (EDL) charges. The ionic species (cations and anions) in the EDL are formed in the electrolyte near an electrode surface (but not on the electrode surface per se) when voltage is imposed upon a symmetric supercapacitor (or EDLC), as schematically illustrated in
When the supercapacitor is re-charged, the ions (both cations and anions) already pre-existing in the liquid electrolyte are formed into EDLs near their respective local electrodes. There is no exchange of ions between an anode active material and a cathode active material. The amount of charges that can be stored (capacitance) is dictated solely by the concentrations of cations and anions that pre-exist in the electrolyte. These concentrations are typically very low and are limited by the solubility of a salt in a solvent, resulting in a low energy density.
In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some electrochemical reactions (e.g., redox). In such a pseudo-capacitor, the ions involved in a redox pair also pre-exist in the electrolyte. Again, there is no exchange of ions between an anode active material and a cathode active material.
Since the formation of EDLs does not involve a chemical reaction or an exchange of ions between the two opposite electrodes, the charge or discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (more typically 3,000-8,000 W/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 20-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Lithium-ion batteries possess a much higher energy density, typically in the range of 100-180 Wh/kg, based on the total cell weight.
Although possessing a much higher energy density, lithium-ion batteries deliver a very low power density (typically 100-500 W/Kg), requiring typically hours for re-charge. Conventional lithium-ion batteries also pose some safety concern.
The low power density or long re-charge time of a lithium ion battery is due to the mechanism of shuttling lithium ions between the interior of an anode and the interior of a cathode, which requires lithium ions to enter or intercalate into the bulk of anode active material particles during re-charge, and into the bulk of cathode active material particles during discharge. For instance, as illustrated in
During discharge, lithium ions diffuse out of the anode active material (e.g. de-intercalate out of graphite particles 10 μm in diameter), migrate through the liquid electrolyte phase, and then diffuse into the bulk of complex cathode crystals (e.g. intercalate into particles lithium cobalt oxide, lithium iron phosphate, or other lithium insertion compound), as illustrated in
In other words, these intercalation or solid-state diffusion processes require a long time to accomplish because solid-state diffusion (or diffusion inside a solid) is difficult and slow. This is why, for instance, the current lithium-ion battery for plug-in hybrid vehicles requires 2-7 hours of recharge time, as opposed to just seconds for supercapacitors. The above discussion suggests that an energy storage device that is capable of storing as much energy as in a battery and yet can be fully recharged in one or two minutes like a supercapacitor would be considered a revolutionary advancement in energy storage technology.
A hybrid energy storage cell that is developed for the purpose of combining some features of an EDL or symmetric supercapacitor and those of a lithium-ion battery (LIB) is a lithium-ion capacitor (LIC). A LIC contains a lithium intercalation compound (e.g., graphite particles) as an anode and an EDL capacitor-type cathode (e.g. activated carbon, AC), as schematically illustrated in
When the LIC is discharged, lithium ions migrate out from the interior of graphite particles (a slow solid-state diffusion process) to enter the electrolyte phase and, concurrently, the counter-ions PF6− are also released from the EDL zone, moving further away from AC surfaces into the bulk of the electrolyte. In other words, both the cations (Li+ ions) and the anions (PF6−) are randomly disposed in the liquid electrolyte, not associated with any electrode (
Furthermore, due to the need to undergo de-intercalation and intercalation at the anode, the power density of a LIC is not high (typically <10 kW/kg, which is comparable to or only slightly higher than those of an EDLC).
Recently, chemically treated multi-walled carbon nano-tubes (CNTs) containing carbonyl groups were used by Lee, et al as a cathode active material for a LIC containing lithium titanate as the anode material [S. W. Lee, et al, “High Power Lithium Batteries from Functionalized Carbon Nanotubes,” Nature Nanotechnology, 5 (2010) 531-537]. This is another type of hybrid battery/supercapacitor device or lithium-ion capacitor. In addition, in a half-cell configuration discussed in the same report, lithium foil was used by Lee, et al as the anode and functionalized CNTs as the cathode, providing a relatively high power density. However, the CNT-based electrodes prepared by the layer-by-layer (LBL) approach suffer from several technical issues beyond just the high costs. Some of these issues are:
Most recently, our research group has invented a revolutionary class of high-power and high-energy-density energy storage devices now commonly referred to as the surface-mediated cell (SMC). This has been reported in the following patent applications and a scientific paper:
In a fully surface-mediated cell, f-SMC, as illustrated in
When the SMC cell is made, particles or foil of lithium metal are implemented at the anode (
A particularly useful nano-structured electrode material is nano graphene platelet (NGP), which refers to either a single-layer graphene sheet or multi-layer graphene pletelets. A single-layer graphene sheet is a 2-D hexagon lattice of carbon atoms covalently bonded along two plane directions. We have studied a broad array of graphene materials for electrode uses: pristine graphene, graphene oxide, chemically or thermaly reduced graphene, graphene fluoride, chemically modified graphene, hydrogenated graphene, nitrogenated graphene, doped graphene. In all cases, both single-layer and multi-layer graphene were prepared from natural graphite, petroleum pitch-derived artificial graphite, micron-scaled graphite fibers, activated carbon (AC), and treated carbon black (t-CB). AC and CB contain narrower graphene sheets or aromatic rings as a building block, while graphite and graphite fibers contain wider graphene sheets. Their micro-structures all have to be exfoliated (to increase inter-graphene spacing in graphite) or activated (to open up nano gates or pores in t-CB) to allow liquid electrolyte to access more graphene edges and surfaces where lithium can be captured. Other types of disordered carbon studied have included soft carbon (including meso-phase carbon, such as meso-carbon micro-beads), hard carbon (including petroleum coke), and amorphous carbon, in addition to carbon black and activated carbon. All these carbon/graphite materials have graphene sheets dispersed in their microstructure.
These highly conducting materials, when used as a cathode active material, can have a functional group that is capable of rapidly and reversibly forming a redox reaction with lithium ions. This is one possible way of capturing and storing lithium directly on a graphene surface (including edge). We have also discovered that the benzene ring centers of graphene sheets are highly effective and stable sites for capturing and storing lithium atoms, even in the absence of a lithium-capturing functional group.
Similarly, in a lithium super-battery (p-SMC), the cathode includes a chemically functionalized NGP or a functionalized disordered carbon material having certain specific functional groups capable of reversibly and rapidly forming/releasing a redox pair with a lithium ion during the discharge and charge cycles of a p-SMC. In a p-SMC, the disordered carbon or NGP is used in the cathode (not the anode) of the lithium super-battery. In this cathode, lithium ions in the liquid electrolyte only have to migrate to the edges or surfaces of graphene sheets (in the case of functionalized NGP cathode), or the edges/surfaces of the aromatic ring structures (small graphene sheets) in a disordered carbon matrix. No solid-state diffusion is required at the cathode. The presence of a functionalized graphene or carbon having functional groups thereon enables reversible storage of lithium on the surfaces (including edges), not the bulk, of the cathode material. Such a cathode material provides one type of lithium-storing or lithium-capturing surface. Again, another possible mechanism is based on the benzene ring centers of graphene sheets that are highly effective and stable sites for capturing and storing lithium atoms.
In a lithium super-battery or p-SMC, the anode comprises a current collector and a lithium foil alone (as a lithium source), without an anode active material to support or capture lithium ions/atoms. Lithium has to deposit onto the front surface of an anode current collector alone (e.g. copper foil) when the battery is re-charged. Since the specific surface area of a current collector is very low (typically <1 m2/gram), the over-all lithium re-deposition rate can be relatively low as compared to f-SMC.
The features and advantages of SMCs that differentiate the SMC from conventional lithium-ion batteries (LIB), supercapacitors, and lithium-ion capacitors (LIC) are summarized below:
The amount of lithium stored in the lithium source when a SMC is made dictates the amount of lithium ions that can be exchanged between an anode and a cathode. This, in turn, dictates the energy density of the SMC.
In all of the aforementioned electrochemical energy storage devices (supercapacitor, LIB, LIC, p-SMC, f-SMC, and other lithium metal cells, such as lithium-sulfur cell and lithium-air cell), every individual electrode is a single-functional electrode. For instance, the anode in a LIB or LIC is an intercalation compound (e.g. graphite or lithium titanate particles) that stores lithium in the interior or bulk of the compound and the lithium in-take and release depends upon intercalation and de-intercalation of lithium (solid-state diffusion). The cathode (e.g. lithium iron phosphate or lithium cobalt oxide) is also an intercalation compound that stores lithium in the interior of a cathode particle. This type of electrode is herein referred to as an “intercalation electrode active material” or simply “intercalation material.”
In contrast, the cathode active material in a p-SMC or f-SMC (e.g. graphene) operates by capturing and storing lithium atoms on graphene surfaces, requiring no intercalation or de-intercalation. This type of material is herein referred to as an “intercalation-free electrode active material” or “intercalation-free material.”
Every individual electrode (anode or cathode) in all of the known electrochemical energy storage devices is either an intercalation type or an intercalation-free type, but not both. During the course of our investigation on SMC cells, we have discovered a new type of electrode that is herein referred to as a hybrid electrode. A hybrid electrode is composed of at least one intercalation electrode active material and one intercalation-free electrode active material that co-exist in the same electrode, e.g. an interaction material coated on one surface of a current collector and an intercalation-free material coated on an opposing surface of the same current collector. Such a hybrid electrode, when used as an anode and/or as a cathode of an energy storage device, imparts many unique, novel, and unexpected effect to the device.
The present invention provides a multi-component hybrid electrode for use in an electrochemical super-hybrid energy storage device. The electrode itself is a hybrid electrode, not just the energy storage device.
The hybrid electrode contains at least a current collector, at least an intercalation electrode active material storing lithium inside interior or bulk thereof, and at least an intercalation-free electrode active material having a specific surface area no less than 100 m2/g and storing lithium on a surface thereof, wherein the intercalation electrode active material and the intercalation-free electrode active material are in electronic contact with the current collector.
The “intercalation electrode active material” refers to an electrode material that stores lithium in the interior or bulk of the compound. For instance, graphite or lithium titanate particles commonly used in a LIB or LIC are intercalation compounds that store lithium in the interior or bulk of the compound. The insertion and release of lithium normally occur through lithium solid-state diffusion procedures called “intercalation” and “de-intercalation,” respectively. Commonly used cathode active materials in a LIB (e.g. lithium iron phosphate and lithium cobalt oxide) are also intercalation compounds that store lithium in the interior of a cathode particle. Any of these electrode active materials may be selected as an intercalation electrode active material for use in the presently disclosed hybrid electrode. Graphite and carbon-based intercalation compounds, particularly those used in an anode of a LIB, normally have a specific surface area less than 100 m2/g, more typically less than 50 m2/g, and most typically less than 10 m2/g. The LIB industry prefers to use an anode active material less than 3 m2/g due to the concern that a higher specific surface area tends to form a greater amount of solid-electrolyte interphase (SEI) at the anode, irreversibly consuming more lithium. SEI is a highly undesirable feature in a LIB since it is a primary source of capacity irreversibility.
In contrast, the cathode active material in a p-SMC or f-SMC (e.g. graphene) operates by capturing and storing lithium atoms on graphene surfaces, requiring no intercalation or de-intercalation. This type of material is herein referred to as an “intercalation-free electrode active material.”
In a preferred embodiment, the intercalation electrode active material and the intercalation-free electrode active material in a multi-component hybrid electrode form two separate discrete layers that are respectively bonded to two opposing surfaces of the current collector to form a laminated three-layer electrode. Alternatively, they can form two layers stacked together having one layer bonded to a surface of the current collector to form a laminated electrode. Further alternatively, the intercalation electrode active material and the intercalation-free electrode active material may be mixed to form a hybrid active material coated onto one surface or two opposing surfaces of the current collector. Preferably, the current collector is porous to enable passage of lithium ions.
In a desired embodiment, the multi-component hybrid electrode can have at least two current collectors internally connected in parallel, wherein the intercalation electrode active material is coated on at least a surface of a first current collector and the intercalation-free electrode active material is coated on at least a surface of a second current collector.
Preferably, the hybrid electrode is pre-lithiated, having lithium inserted into interior of the intercalation electrode active material and/or having lithium deposited on a surface of the intercalation-free electrode active material before or when the device is made.
It is desirable to have an intercalation electrode active material having a specific surface area less than 100 m2/g. Further desirably, the intercalation electrode active material has a specific surface area less than 100 m2/g and the intercalation-free electrode active material has a specific surface area no less than 500 m2/g. Most desirably, the intercalation electrode active material has a specific surface area less than 50 m2/g and the intercalation-free electrode active material has a specific surface area no less than 1,500 m2/g.
In one possible super-hybrid energy storage device, the hybrid electrode is an anode and the constituent intercalation material is an anode active material selected from the following:
The multi-component hybrid electrode may be used as a cathode, wherein the intercalation material is a cathode active material capable of storing lithium in interior or bulk of the material. The intercalation material can be any element or compound that is used in a conventional lithium ion battery, lithium metal battery, and lithium-sulfur battery.
Preferably, the intercalation material in a hybrid cathode may be selected from the group consisting of lithium cobalt oxide, cobalt oxide, lithium nickel oxide, nickel oxide, lithium manganese oxide, vanadium oxide V2O5, V3O8, lithium transition metal oxide, lithiated oxide of transition metal mixture, non-lithiated oxide of a transition metal, non-lithiated oxide of transition metal mixture, lithium iron phosphate, lithium vanadium phosphate, lithium manganese phosphate, a non-lithiated transition metal phosphate, a chalcogen compound, sulfur, sulfur-containing molecule, sulfur-containing compound, sulfur-carbon polymer, sulfur dioxide, thionyl chloride (SOCl2), oxychloride, manganese dioxide, carbon monofluoride ((CF)n), iron disulfide, copper oxide, lithium copper oxyphosphate (Cu4O(PO4)2), silver vanadium oxide, MoS2, TiS2, NbSe3, and combinations thereof. The intercalation material in such a hybrid cathode can be in a form of nano-scaled particle, wire, rod, tube, ribbon, sheet, film, or coating having a dimension less than 100 nm, preferably less than 20 nm, and most preferably less than 10 nm.
The intercalation-free electrode material may be a cathode active material that forms a porous structure having a specific surface area no less than 100 m2/g, and may be selected from: (a) a porous disordered carbon material selected from activated soft carbon, activated hard carbon, activated polymeric carbon or carbonized resin, activated meso-phase carbon, activated coke, activated carbonized pitch, activated carbon black, activated carbon, or activated partially graphitized carbon; (b) a graphene material selected from a single-layer graphene, multi-layer graphene, graphene oxide, graphene fluoride, hydrogenated graphene, nitrogenated graphene, boron-doped graphene, nitrogen-doped graphene, functionalized graphene, or reduced graphene oxide; (c) a meso-porous exfoliated graphite; (d) a meso-porous carbon; (e) a carbon nanotube (CNT) selected from a single-walled carbon nanotube or multi-walled carbon nanotube, oxidized CNT, fluorinated CNT, hydrogenated CNT, nitrogenated CNT, boron-doped CNT, nitrogen-doped CNT, or doped CNT; (f) a carbon nano-fiber, metal nano-wire, metal oxide nano-wire or fiber, or conductive polymer nano-fiber, or (g) a combination thereof.
The present invention also provides a super-hybrid energy storage device comprising a multi-component hybrid electrode as discussed above. In other words, the super-hybrid device has an electrode (an anode or cathode) that can perform two mechanisms of lithium storage: lithium storage in the interior of an intercalation active material and lithium storage on the surface of an intercalation-free active material. The counter-electrode (a cathode or anode) can be a regular electrode (performing one function only, either intercalation or intercalation-free, but not both) or a hybrid electrode (performing both functions).
In one preferred embodiment, this super-hybrid device contains such a hybrid electrode as an anode, a cathode formed of a porous cathode active material having a specific surface area no less than 100 m2/g in direct contact with electrolyte, a separator disposed between the anode and the cathode, electrolyte in ionic contact with the two electrodes, and at least a lithium source disposed at the anode or cathode prior to the first discharge or charge operation of the device. The super-hybrid device operates on an exchange of lithium ions between a surface and/or interior of an anode active material and a surface of the cathode active material. The cathode active material in this case is itself essentially an intercalation-free active material and can be any cathode active material commonly used in s surface-mediated cell, such as (a) a porous disordered carbon material; (b) a graphene material; (c) a meso-porous exfoliated graphite; (d) a meso-porous carbon; (e) a carbon nanotube (CNT); or (f) a carbon nano-fiber, metal nano-wire, metal oxide nano-wire or fiber, or conductive polymer nano-fiber.
Another embodiment is a super-hybrid energy storage device comprising an anode, a hybrid electrode as a cathode, a separator disposed between the anode and the cathode, electrolyte in ionic contact with the anode and the cathode, and at least a lithium source disposed at the anode or cathode prior to the first discharge or charge of the device. The device operates on an exchange of lithium ions between a surface and/or interior of a cathode active material and a surface of the anode (surface of an anode current collector or anode active material) or interior of an anode active material, if present.
Yet another embodiment is a super-hybrid energy storage device comprising an anode having a current collector and an anode active material, a hybrid electrode as a cathode, a separator disposed between the anode and the cathode, electrolyte in ionic contact with the anode and cathode, and at least a lithium source disposed at the anode or cathode prior to the first discharge or charge of the device. The device operates on the exchange of lithium ions between a surface and/or interior of a cathode active material and a surface of the anode current collector or a surface or interior of the anode active material.
Still another embodiment is a super-hybrid energy storage device comprising a hybrid electrode as an anode, another hybrid electrode as a cathode, a separator disposed between the anode and the cathode, electrolyte in ionic contact with the anode and the cathode, and at least a lithium source disposed at the anode or cathode prior to a first discharge or charge of the device. Both the anode and the cathode can perform two functions (surface storage and bulk storage of lithium). Hence, the device operates on the exchange of lithium ions between a surface and/or interior of a cathode active material and a surface and/or interior of an anode active material.
A particularly desired super-hybrid energy storage device contains two cells internally connected in parallel (having at least one cell being a super-hybrid cell). The device contains: (A) a first anode being formed of a first anode current collector having a surface area to capture or store lithium thereon; (B) a first hybrid cathode comprising a first cathode current collector, a first intercalation-free cathode active material coated on at least a surface of the first cathode current collector, and a first interaction cathode active material coated on a surface of a second cathode current collector, wherein the first and second cathode current collectors are internally connected in parallel; (C) a first porous separator disposed between the first hybrid cathode and the first anode; (D) a lithium-containing electrolyte in physical contact with the first hybrid cathode and first anode; and (E) at least a lithium source implemented at or near at least one of the anodes or cathodes prior to the first charge or first discharge cycle of the energy storage device. Here, the first intercalation-free cathode active material has a specific surface area of no less than 100 m2/g being in direct physical contact with the electrolyte to receive lithium ions therefrom, or to provide lithium ions thereto. Preferably, this super-hybrid energy storage device further comprises a second anode being formed of a second anode current collector having a surface area to capture or store lithium thereon. Preferably, the first anode contains an anode active material having a specific surface area greater than 100 m2/g. In general, the first anode current collector and the second anode current collector are connected to an anode terminal, and the first cathode current collector and the second cathode current collector are connected to a cathode terminal.
The device can be composed of at least two cells with one cell being a super-hybrid cell (having at least a hybrid electrode as an anode or a cathode) and the other cell either a regular intercalation-dominated cell (both the anode and the cathode operating essentially on lithium intercalation and de-intercalation) or a regular intercalation-free cell (surface-mediated cell). It is also desirable to have both two cells being super-hybrid cells (each cell having at least a hybrid electrode).
It is desirable to have at least one of the anode current collectors or cathode current collectors being a porous, electrically conductive material selected from metal foam, metal web or screen, perforated metal sheet, metal fiber mat, metal nanowire mat, porous conductive polymer film, conductive polymer nano-fiber mat or paper, conductive polymer foam, carbon foam, carbon aerogel, carbon xerox gel, graphene foam, graphene oxide foam, reduced graphene oxide foam, carbon fiber paper, graphene paper, graphene oxide paper, reduced graphene oxide paper, carbon nano-fiber paper, carbon nano-tube paper, or a combination thereof.
In a super-hybrid device, at least one of the cells contains therein a lithium source prior to a first charge or a first discharge cycle of the energy storage device. The lithium source may be preferably in a form of solid lithium foil, lithium chip, lithium powder, or surface-stabilized lithium particles. The lithium source may be a layer of lithium thin film pre-loaded on surfaces of an electrode active material or a current collector. In one preferred embodiment, the entire device has just one lithium source. Preferably, the lithium source is a lithium thin film or coating pre-plated on the surface of an anode current collector or anode active material, or simply a sheet of lithium foil implemented near or on a surface of an anode current collector or anode active material.
The surfaces of a hybrid electrode material in a super-hybrid cell or an intercalation-free material in a SMC are capable of capturing lithium ions directly from a liquid electrolyte phase and storing lithium atoms on the surfaces in a reversible and stable manner. The electrolyte preferably comprises liquid electrolyte (e.g. organic liquid or ionic liquid) or gel electrolyte in which lithium ions have a high diffusion coefficient. Solid electrolyte is normally not desirable, but some thin layer of solid electrolyte may be used if it exhibits a relatively high diffusion rate.
The present invention provides a multi-component hybrid electrode for use in an electrochemical super-hybrid energy storage device. The hybrid electrode itself is a hybrid of two electrode materials and, hence, an energy storage cell containing a hybrid electrode is herein referred to as a super-hybrid cell.
The electrode in a conventional lithium-ion battery is normally a single-functional electrode performing either an intercalation-based lithium storage mechanism (storing lithium in the interior of an electrode active material) or an intercalation-free mechanism (storing lithium on the surface of an electrode active material), but not both.
Schematically shown in
A preferred embodiment of the present invention, as schematically shown in
Useful graphene-rich carbon materials include: (a) a porous disordered carbon material selected from activated soft carbon, activated hard carbon, activated polymeric carbon or carbonized resin, activated meso-phase carbon, activated coke, activated carbonized pitch, activated carbon black, activated carbon, or activated partially graphitized carbon; (b) a graphene material selected from a single-layer graphene, multi-layer graphene, graphene oxide, graphene fluoride, hydrogenated graphene, nitrogenated graphene, boron-doped graphene, nitrogen-doped graphene, functionalized graphene, or reduced graphene oxide; (c) a meso-porous exfoliated graphite; (d) a meso-porous carbon; (e) a carbon nanotube (CNT) selected from a single-walled carbon nanotube or multi-walled carbon nanotube, oxidized CNT, fluorinated CNT, hydrogenated CNT, nitrogenated CNT, boron-doped CNT, nitrogen-doped CNT, or doped CNT; and (f) a carbon nano-fiber. These nano-structured carbon materials contain some graphene sheets, small or large, as a constituent ingredient. For instance, a single-wall CNT is essentially a layer of graphene rolled up into a tubular shape. The disordered carbon must be chemically or physically activated, or exfoliated to produce meso-scaled pores (>2 nm) and/or expanding the inter-graphene spacing to >2 nm, allowing liquid electrolyte to access graphene surfaces.
According to another preferred embodiment of the present invention, a hybrid electrode can contain a layer of intercalation-free active material and a layer of intercalation active material respectively bonded to two opposing surfaces of an electrode current collector, as illustrated in
The porous current collector can be an electrically conductive material that forms a porous structure (preferably meso-porous having a pore size in the range of 2 nm and 50 nm). This conductive material may be selected from metal foam, metal web or screen, perforated metal sheet (having pores penetrating from a front surface to a back surface), metal fiber mat, metal nanowire mat, porous conductive polymer film, conductive polymer nano-fiber mat or paper, conductive polymer foam, carbon foam, carbon aerogel, carbon xerox gel, graphene foam, graphene oxide foam, reduced graphene oxide foam, carbon fiber paper, graphene paper, graphene oxide paper, reduced graphene oxide paper, carbon nano-fiber paper, carbon nano-tube paper, or a combination thereof. These materials can be readily made into an electrode that is porous (preferably having a specific surface area greater than 50 m2/g, more preferably >100 m2/g, further preferably >500 m2/g, even more preferably >1,000 m2/g, and most preferably >1,500 m2/g), allowing liquid electrolyte and the lithium ions contained therein to migrate through.
In an alternative configuration, a hybrid electrode can be composed of two or more current collectors internally connected in parallel, wherein at least one current collector having an intercalation active material coated thereon and at least one current collector having an intercalation-free active material coated thereon.
For use in a cathode, the intercalation electrode active material of a hybrid electrode may be selected from a broad range of cathode active materials that are capable of storing lithium in interior or bulk of the material. The intercalation material can be any element or compound used in a conventional lithium ion battery, lithium metal battery, and lithium-sulfur battery.
Preferably, the intercalation material in a hybrid cathode (a hybrid electrode used as a cathode) may be selected from the group consisting of lithium cobalt oxide, cobalt oxide, lithium nickel oxide, nickel oxide, lithium manganese oxide, vanadium oxide V2O5, V3O8, lithium transition metal oxide, lithiated oxide of transition metal mixture, non-lithiated oxide of a transition metal, non-lithiated oxide of transition metal mixture, lithium iron phosphate, lithium vanadium phosphate, lithium manganese phosphate, a non-lithiated transition metal phosphate, a chalcogen compound, sulfur, sulfur-containing molecule, sulfur-containing compound, sulfur-carbon polymer, sulfur dioxide, thionyl chloride (SOCl2), oxychloride, manganese dioxide, carbon monofluoride ((CF)n), iron disulfide, copper oxide, lithium copper oxyphosphate (Cu4O(PO4)2), silver vanadium oxide, MoS2, TiS2, NbSe3, and combinations thereof. The intercalation material in such a hybrid cathode can be in a form of nano-scaled particle, wire, rod, tube, ribbon, sheet, film, or coating having a dimension less than 100 nm, preferably less than 20 nm, and most preferably less than 10 nm.
For use in an anode, the intercalation active material of a hybrid electrode may be selected from the following: (A) a graphite or carbonaceous intercalation compound having a specific surface area less than 100 m2/g (preferably less than 50 m2/g, further preferably less than 10 m2/g) when formed into an anode (e.g. the intercalation compound may be selected from natural graphite, synthetic graphite, meso-phase carbon, soft carbon, hard carbon, amorphous carbon, polymeric carbon, coke, meso-porous carbon, carbon fiber, graphite fiber, carbon nano-fiber, carbon nano-tube, and expanded graphite platelets or nano graphene platelets containing multiple graphene planes bonded together); (B) silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), and cadmium (Cd); (C) alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, or Cd with other elements, wherein said alloys or compounds are stoichiometric or non-stoichiometric; (D) oxides, carbides, nitrides, sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Ni, Co, Mn, Fe, or Cd, and their mixtures, composites, or lithium-containing composites, including Co3O4, Mn3O4, and their mixtures or composites; (E) salts and hydroxides of Sn; (F) lithium titanate, lithium manganate, lithium aluminate, lithium-containing titanium oxide, lithium transition metal oxide; or (G) a combination thereof.
The present invention also provides a super-hybrid cell containing at least a hybrid electrode as an anode or a cathode. As schematically illustrated in
In a preferred embodiment, a super-hybrid cell can contain a hybrid anode and a hybrid cathode. Further alternatively, a super-hybrid device may contain a hybrid electrode that is formed of two current collectors internally connected in parallel with one current collector supporting at least a layer of intercalation active material and the other current collector supporting at least a layer of intercalation-free active material
The lithium source in a super-hybrid cell preferably comprises a lithium chip, lithium foil, lithium powder, surface stabilized lithium particles, lithium film coated on a surface of an anode or cathode current collector, lithium film coated on a surface of an anode or cathode active material, or a combination thereof. Coating of lithium on the surfaces of a current collector or an electrode can be accomplished via electrochemical deposition (plating), sputtering, vapor deposition, etc. Preferably, at least one of the anode current collectors or at least one of the cathode active materials is pre-loaded (pre-lithiated, pre-coated, or pre-plated) with lithium before or when the stack is made.
The electrolyte is preferably liquid electrolyte or gel electrolyte containing a first amount of lithium ions dissolved therein. The operation of an SMC cell or a super-hybrid cell involves an exchange of a second amount of lithium ions between the cathodes and the anodes, and this second amount of lithium is greater than the first amount.
Although there is no limitation on the electrode thickness, the active material layer coated on a current collector in a presently invented hybrid electrode preferably has a thickness greater than 5 μm, more preferably greater than 50 μm, and most preferably greater than 100 μm.
Another preferred embodiment of the present invention is a stack of electrochemical cells that are internally connected in series or in parallel, containing at least one hybrid electrode.
The invention further provides a super-hybrid energy storage device, which is internally connected to an electrochemical energy storage device in series or in parallel, wherein the electrochemical energy storage device is selected from a supercapacitor, a lithium-ion capacitor, a lithium-ion battery, a lithium metal secondary battery, a lithium-sulfur cell, a surface-mediated cell (f-SMC or p-SMC), or another super-hybrid cell containing a hybrid electrode.
The operation of a super-hybrid cell may be illustrated in
During the first discharge of this super-hybrid cell, lithium foil is ionized, releasing lithium ions into electrolyte, penetrating through the porous anode current collector and porous anode active material layers, migrating through the porous separator, reaching the cathode side through liquid electrolyte, and get captured by the surfaces of an intercalation-free cathode active material (
When this super-hybrid cell is re-charged, massive lithium ions are released immediately from the surfaces of a cathode active material having a high specific surface area. Under the influence of an electric field generated by an outside battery charger, lithium ions are driven to swim in liquid electrolyte through the porous separator and reach the anode side. With a hybrid anode, some of the lithium ions can get captured by surfaces of the intercalation-free active materials (e.g. graphene or meso-porous carbon) in a short period of time. The remaining lithium ions will take time to intercalate into the interior of graphite particles.
This new super-hybrid cell has an intercalation-free electrode, similar to what is used in a surface-mediated cell (SMC). However, this super-hybrid cell has several unique and novel properties that are not found with the SMC or any other electrochemical energy storage device, as demonstrated in the Examples. In addition, the super-hybrid cell is also patently distinct from the conventional supercapacitor in the following aspects:
Our earlier studies [Ref. 1-6 cited earlier] have established that the specific capacity of an intercalation-free electrode in a SMC is governed by the number of active sites on graphene surfaces of a nano-structured carbon material that are capable of capturing lithium ions thereon, as illustrated in
Single-layer graphene or the graphene plane (a layer of carbon atoms forming a hexagonal or honeycomb-like structure) is a common building block of a wide array of graphitic materials, including natural graphite, artificial graphite, soft carbon, hard carbon, coke, activated carbon, carbon black, etc. In these graphitic materials, typically multiple graphene sheets are stacked along the graphene thickness direction to form an ordered domain or crystallite of graphene planes. Multiple crystallites of domains are then connected with disordered or amorphous carbon species. In the instant application, we are able to extract or isolate these crystallites or domains to obtain multiple-layer graphene platelets out of the disordered carbon species. In some cases, we exfoliate and separate these multiple-graphene platelets into isolated single-layer graphene sheets. In other cases (e.g. in activated carbon, hard carbon, and soft carbon), we chemically removed some of the disordered carbon species to open up gates, allowing liquid electrolyte to enter into the interior (exposing graphene surfaces to electrolyte).
In the present application, nano graphene platelets (NGPs) or “graphene materials” collectively refer to single-layer and multi-layer versions of graphene, graphene oxide, graphene fluoride, hydrogenated graphene, nitrogenated graphene, doped graphene, boron-doped graphene, nitrogen-doped graphene, etc.
The disordered carbon material may be selected from a broad array of carbonaceous materials, such as a soft carbon, hard carbon, polymeric carbon (or carbonized resin), meso-phase carbon, coke, carbonized pitch, carbon black, activated carbon, or partially graphitized carbon. A disordered carbon material is typically formed of two phases wherein a first phase is small graphite crystal(s) or small stack(s) of graphite planes (with typically up to 10 graphite planes or aromatic ring structures overlapped together to form a small ordered domain) and a second phase is non-crystalline carbon, and wherein the first phase is dispersed in the second phase or bonded by the second phase. The second phase is made up of mostly smaller molecules, smaller aromatic rings, defects, and amorphous carbon. Typically, the disordered carbon is highly porous (e.g., activated carbon) or present in an ultra-fine powder form (e.g. carbon black) having nano-scaled features (hence, a high specific surface area).
Soft carbon refers to a carbonaceous material composed of small graphite crystals wherein the orientations of these graphite crystals or stacks of graphene sheets are conducive to further merging of neighboring graphene sheets or further growth of these graphite crystals or graphene stacks using a high-temperature heat treatment (graphitization). Hence, soft carbon is said to be graphitizable. Hard carbon refers to a carbonaceous material composed of small graphite crystals wherein these graphite crystals or stacks of graphene sheets are not oriented in a favorable directions (e.g. nearly perpendicular to each other) and, hence, are not conducive to further merging of neighboring graphene sheets or further growth of these graphite crystals or graphene stacks (i.e., not graphitizable).
Carbon black (CB), acetylene black (AB), and activated carbon (AC) are typically composed of domains of aromatic rings or small graphene sheets, wherein aromatic rings or graphene sheets in adjoining domains are somehow connected through some chemical bonds in the disordered phase (matrix). These carbon materials are commonly obtained from thermal decomposition (heat treatment, pyrolyzation, or burning) of hydrocarbon gases or liquids, or natural products (wood, coconut shells, etc). The preparation of polymeric carbons by simple pyrolysis of polymers or petroleum/coal tar pitch materials has been known for approximately three decades. When polymers such as polyacrylonitrile (PAN), rayon, cellulose and phenol formaldehyde were heated above 300° C. in an inert atmosphere they gradually lost most of their non-carbon contents. The resulting structure is generally referred to as a polymeric carbon.
Polymeric carbons can assume an essentially amorphous structure, or have multiple graphite crystals or stacks of graphene planes dispersed in an amorphous carbon matrix. Depending upon the HTT used, various proportions and sizes of graphite crystals and defects are dispersed in an amorphous matrix. Various amounts of two-dimensional condensed aromatic rings or hexagons (precursors to graphene planes) can be found inside the microstructure of a heat treated polymer such as a PAN fiber. An appreciable amount of small-sized graphene sheets are believed to exist in PAN-based polymeric carbons treated at 300-1,000° C. These species condense into wider aromatic ring structures (larger-sized graphene sheets) and thicker plates (more graphene sheets stacked together) with a higher HTT or longer heat treatment time (e.g., >1,500° C.). These graphene platelets or stacks of graphene sheets (basal planes) are dispersed in a non-crystalline carbon matrix. Such a two-phase structure is a characteristic of some disordered carbon material.
Certain grades of petroleum pitch or coal tar pitch may be heat-treated (typically at 250-500° C.) to obtain a liquid crystal-type, optically anisotropic structure commonly referred to as meso-phase. This meso-phase material can be extracted out of the liquid component of the mixture to produce meso-phase particles or spheres, which can be carbonized and optionally graphitized. A commonly used meso-phase carbon material is referred to as meso-carbon micro-beads (MCMBs).
Physical or chemical activation may be conducted on all kinds of disordered carbon (e.g. a soft carbon, hard carbon, polymeric carbon or carbonized resin, meso-phase carbon, coke, carbonized pitch, carbon black, activated carbon, or partially graphitized carbon) to obtain activated disordered carbon. For instance, the activation treatment can be accomplished through oxidizing, CO2 physical activation, KOH or NaOH chemical activation, or exposure to nitric acid, fluorine, or ammonia plasma (for the purpose of creating electrolyte-accessible pores, not for functionalization).
The following examples serve to illustrate the preferred embodiments of the present invention and should not be construed as limiting the scope of the invention:
Soft carbon materials were prepared from a liquid crystalline aromatic resin. The resin was ground with a mortar, and calcined at 900° C. for 2 h in a N2 atmosphere to prepare the graphitizable carbon or soft carbon. The resulting soft carbon was mixed with small tablets of KOH (four-fold weight) in an alumina melting pot. Subsequently, the soft carbon containing KOH was heated at 750° C. for 2 h in N2. Upon cooling, the alkali-rich residual carbon was washed with hot water until the outlet water reached a pH value of 7. The resulting material is activated soft carbon.
Coin cells were made that contain activated soft carbon as a cathode intercalation-free material and LiCO2 as an intercalation cathode active material, activated soft carbon as a nano-structured anode, and a thin piece of lithium foil as a lithium source implemented between a current collector and a separator layer. Corresponding SMC cells without LiCO2 were also prepared and tested for comparison. In all cells, the separator used was one sheet of micro-porous membrane (Celgard 2500). The current collector for each of the two cathodes was a piece of porous carbon-coated aluminum foil.
For the super-hybrid cell, the front surface (facing the separator) of the porous cathode current collector was coated with activated soft carbon layer composed of a composite composed of 85 wt. % activated soft carbon (+5% Super-P and 10% PTFE binder). The back surface was coated with a composite layer composed of 85 wt. % LiCO2 (+5% Super-P and 10% PTFE binder). The electrolyte solution was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) with a 3:7 volume ratio. The separator was wetted by a minimum amount of electrolyte to reduce the background current. Cyclic voltammetry and galvanostatic measurements of the lithium cells were conducted using an Arbin 32-channel supercapacitor-battery tester at room temperature (in some cases, at a temperature as low as −40° C. and as high as 60° C.).
As a reference sample, a similar Lithium-ion cell having a natural graphite-based intercalation anode active material and LiCO2 cathode was made and tested. Additionally, a symmetric supercapacitor with both electrodes being composed of an activated soft carbon material, but containing no additional lithium source than what is available in the liquid electrolyte, was also fabricated and evaluated.
Galvanostatic studies of these four samples have enabled us to obtain significant data as summarized in the Ragone plot of
Natural graphite (HuaDong Graphite Co., Qingdao, China) having a median size of about 45 microns and an inter-planar distance of about 0.335 nm was intercalated with an acid solution (sulfuric acid, nitric acid, and potassium permanganate at a ratio of 4:1:0.05) for 72 hours. Upon completion of the reaction, the mixture was poured into deionized water and filtered. The intercalated graphite or oxidized graphite was repeatedly washed in a 5% solution of HCl to remove most of the sulphate ions. The sample was then washed repeatedly with deionized water until the pH of the filtrate was neutral. The slurry was dried and stored in a vacuum oven at 60° C. for 24 hours. The dried powder sample was placed in a quartz tube and inserted into a horizontal tube furnace pre-set at a desired temperature, 1,050° C. for 45 seconds to obtain exfoliated graphite. Isolated NGPs were then obtained via ultrasonication of exfoliated graphite in water, forming a graphene-water suspension.
For the preparation of a SMC, NGPs were used as an intercalation-free cathode active material and an activated soft carbon was used as an intercalation-free anode material. A lithium foil was added between the anode and the separator.
For the preparation of vanadium oxide-based intercalation cathode active material, V2O5 (99.6%, Alfa Aesar) and LiOH (99+%, Sigma-Aldrich) were used to prepare the precursor solution. Graphene (1% w/v obtained above) was used as a structure modifier. First, V2O5 and LiOH in a stoichiometric V/Li ratio of 1:3 were dissolved in actively stirred de-ionized water at 50° C. until an aqueous solution of LixV3O8 was formed. Then, graphene-water suspension was added while stirring, and the resulting suspension was atomized and dried in an oven at 160° C. to produce the spherical composite particulates of graphene/LixV3O8 nano-sheets (graphene-wrapped LixV3O8 particles). In a conventional lithium metal secondary cell (as a control sample), lithium foil was used as an anode active material and these composite particles were used as a cathode active material.
A super-hybrid cell was made that was formed of an NGP anode (intercalation-free) and a hybrid cathode composed of an intercalation-free NGP layer bonded to one surface of a cathode current collector and a graphene-wrapped LixV3O8 composite layer bonded to the opposing layer of the cathode current collector.
The Ragone plots for these three cells are shown in
Meso-phase carbon was carbonized at 500° C. for 3 hours and then heat treated at 1500° C. for 4 hours to obtain meso-carbon, which was powderized to obtain meso-carbon particles typically 5-34 μm in size. Meso-carbon particles were mixed with small tablets of KOH (four-fold weight) in an alumina melting pot. Subsequently, the carbon-KOH mixture was heated at 850° C. for 2 h in N2. Upon cooling, the alkali-rich residual carbon was washed with hot water until the outlet water reached a pH value of 7. The resulting material is activated meso-porous carbon. Four cells were prepared and tested:
The Ragone plots of these four cells are shown in
Also surprisingly, the presence of 30% graphite (intercalation compound) in a hybrid anode of the super-hybrid cell does not have any negative impact on the electrochemical performance. One would expect that the presence of graphite that requires intercalation would slow down the charge-discharge process significantly. Contrary to what one would expect, this did not happen. In addition, as illustrated in
For the preparation of a Li—S cell, a cathode film was made by mixing 50% by weight of elemental sulfur, 13% graphene, polyethylene oxide (PEO), and lithium trifluoro-methane-sulfonimide (wherein the concentration of the electrolyte salt to PEO monomer units (CH2CH2O) per molecule of salt was 99:1], and 5% 2,5-dimercapto-1,3,4-dithiadiazole in a solution of acetonitrile (the solvent to PEO ratio being 60:1 by weight). The components were stir-mixed for approximately two days until the slurry was well mixed and uniform. A thin cathode film was cast directly onto stainless steel current collectors, and the solvent was allowed to evaporate at ambient temperatures. The resulting graphene-wrapped sulfur particle-based film weighed approximately 0.0030-0.0058 gm/cm2.
The polymeric electrolyte separator was made by mixing PEO with lithium trifluoromethanesulfonimide, (the concentration of the electrolyte salt to PEO monomer units (CH2CH2O) per molecule of salt being 39:1) in a solution of acetonitrile (the solvent to polyethylene oxide ratio being 15:1 by weight). The components were stir-mixed for two hours until the solution was uniform. Measured amounts of the separator slurry were cast into a retainer onto a release film, and the solvent was allowed to evaporate at ambient temperatures. The resulting electrolyte separator film weighed approximately 0.0146-0.032 gm/cm2.
The cathode film and polymeric electrolyte separator were assembled under ambient conditions, and then vacuum dried overnight to remove moisture prior to being transferred into an argon glove box for final cell assembly with a 3 mil (75 micron) thick lithium anode foil. The anode current collector was Cu foil. Once assembled, the cell was compressed at 3 psi and heated at 40° C. for approximately 6 hours to obtain an integral cell structure.
For a super-hybrid cell, a layer of graphene-wrapped sulfur particle film is coated on a surface of a porous cathode current collector and a layer of intercalation-free graphene sheets is coated on the opposing surface.
A super-hybrid energy storage device may be internally connected to an electrochemical energy storage device in series or in parallel, wherein the electrochemical energy storage device may be selected from a supercapacitor, lithium-ion capacitor, lithium-ion battery, lithium metal secondary battery, lithium-sulfur cell, surface-mediated cell, or super-hybrid cell. Alternatively, the super-hybrid energy storage device may be internally connected in series or in parallel to an intercalation or intercalation-free electrode of an electrochemical energy storage device, selected from a supercapacitor, lithium-ion capacitor, lithium-ion battery, lithium metal secondary battery, lithium-sulfur cell, surface-mediated cell, or super-hybrid cell.
The internal parallel connection of multiple cells, including at least a super-hybrid cell, to form a stack provides several unexpected advantages over individual cells that are externally connected in parallel:
The internal parallel connection of multiple cells, including at least a super-hybrid cell, to form a stack has a characteristic that the electrolyte in one cell does not communicate with the electrolyte in another cell. The two cells are electronically connected through a common current collector that is non-porous and non-permeable to liquid electrolyte. The presently invented internal series connection technology has the following additional features and advantages:
In conclusion, the instant invention provides a revolutionary energy storage device that has exceeded the best features of a supercapacitor, a lithium ion battery, a lithium metal rechargeable battery, a Li—S cell, and/or an SMC. The super-hybrid cells are capable of storing an energy density of >300 Wh/kgcell, which is 60 times higher than that of conventional electric double layer (EDL) supercapacitors. The power density of typically 20-100 kW/kgcell is 20-100 times higher than that (1 kW/kgcell) of conventional lithium-ion batteries. These super-hybrid cells can be re-charged in minutes, as opposed to hours for conventional lithium ion batteries. This is truly a major breakthrough and revolutionary technology.
This invention is based on the research results of a project sponsored by the US National Science Foundation SBIR-STTR Program.