FIELD

The present disclosure relates generally to the field of rechargeable multivalent metal battery (e.g. zinc-, nickel-, magnesium-, or calcium-ion battery) and, more particularly, to a high-capacity cathode active layer of exfoliated graphite or carbon worms and a method of manufacturing the multivalent metal-ion battery.

BACKGROUND

Historically, today's most favorite rechargeable energy storage devices—lithium-ion batteries—was actually evolved from rechargeable “lithium metal batteries” that use lithium (Li) metal as the anode and a Li intercalation compound (e.g. MoS2) as the cathode. Li metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). Based on these outstanding properties, lithium metal batteries were proposed 40 years ago as an ideal system for high energy-density applications.

Due to some safety concerns of pure lithium metal, graphite was implemented as an anode active material in place of the lithium metal to produce the current lithium-ion batteries. The past two decades have witnessed a continuous improvement in Li-ion batteries in terms of energy density, rate capability, and safety. However, the use of graphite-based anodes in Li-ion batteries has several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g as opposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g. low solid-state diffusion coefficients of Li in and out of graphite and inorganic oxide particles) requiring long recharge times (e.g. 7 hours for electric vehicle batteries), inability to deliver high pulse power, and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide, as opposed to cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathode active materials have a relatively low lithium diffusion coefficient (typically D˜10−16−10−11cm2/sec). These factors have contributed to one major shortcoming of today's Li-ion batteries—a moderate energy density (typically 150-220 Wh/kgcell), but extremely low power density (typically <0.5 kW/kg).

Supercapacitors are being considered for electric vehicle (EV), renewable energy storage, and modern grid applications. The relatively high volumetric capacitance density of a supercapacitor (10 to 100 times greater than those of electrolytic capacitors) derives from using porous electrodes to create a large surface area conducive to the formation of diffuse double layer charges. This electric double layer capacitance (EDLC) is created naturally at the solid-electrolyte interface when voltage is imposed. This implies that the specific capacitance of a supercapacitor is directly proportional to the specific surface area of the electrode material, e.g. activated carbon. This surface area must be accessible by the electrolyte and the resulting interfacial zones must be sufficiently large to accommodate the EDLC charges.

This EDLC mechanism is based on surface ion adsorption. The required ions are pre-existing in a liquid electrolyte and do not come from the opposite electrode. In other words, the required ions to be deposited on the surface of a negative electrode (anode) active material (e.g., activated carbon particles) do not come from the positive electrode (cathode) side, and the required ions to be deposited on the surface of a cathode active material do not come from the anode side. When a supercapacitor is re-charged, local positive ions are deposited close to a surface of a negative electrode with their matting negative ions staying close side by side (typically via local molecular or ionic polarization of charges). At the other electrode, negative ions are deposited close to a surface of this positive electrode with the matting positive ions staying close side by side. Again, there is no exchange of ions between an anode active material and a cathode active material.

In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some local electrochemical reactions (e.g., redox). In such a pseudo-capacitor, the ions involved in a redox pair also pre-exist in the same electrode. Again, there is no exchange of ions between the anode and the cathode.

Since the formation of EDLC 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 (typically 3-10 kW/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. 10-30 Wh/kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Modern lithium-ion batteries possess a much higher energy density, typically in the range from 150-220 Wh/kg, based on the cell weight.

In addition to lithium-ion cells, there are several other different types of batteries that are widely used in society: alkaline Zn/MnO2, nickel metal hydride (Ni-MH), lead-acid (Pb acid), and nickel-cadmium (Ni—Cd) batteries. Since their disclosure in 1860, alkaline Zn/MnO2batteries have become a highly popular primary (non-rechargeable) battery. It is now known that the Zn/MnO2pair can constitute a rechargeable battery if an acidic salt electrolyte, instead of basic (alkaline) salt electrolyte, is utilized. However, the cycle life of alkaline manganese dioxide rechargeable batteries has been limited to typically 20-30 cycles due to irreversibility associated with MnO2upon deep discharge and formation of electrochemically inactive phases.

Additionally, formation of a haeterolite (ZnO:Mn2O3) phase during discharge, when Zn penetrates into the lattice structure of MnO2, has made battery cycling irreversible. The Zn anode also has limitations on cycle life due to the redistribution of Zn active material and formation of dendrites during recharge, causing internal short-circuits. Attempts to solve some of these issues have been made by Oh, et al. [S. M. Oh, and S. H. Kim, “Aqueous Zinc Sulfate (II) Rechargeable Cell Containing Manganese (II) Salt and Carbon Powder,” U.S. Pat. No. 6,187,475, Feb. 13, 2001] and by Kang, et al. [F. Kang, et al. “Rechargeable Zinc Ion Battery”, U.S. Pat. No. 8,663,844, Mar. 4, 2014]. However, long-term cycling stability and power density issues remain to be resolved. Due to these reasons, the commercialization of this battery has been limited.

Xu, et al. US Pub. No. 2016/0372795 (Dec. 22, 2016) and US Pub. No. 2015/0255792 (Sep. 10, 2015) reported Ni-ion and Zn-ion cells, respectively, which both make use of graphene sheets or carbon nanotubes (CNTs) as the cathode active material. Although these two patent applications claim an abnormally high specific capacity of 789-2500 mAh/g based on the cathode active material weight, there are several serious problems associated with these two cells:(1) There is no plateau portion in the charge or discharge curves (voltage vs. time or voltage vs. specific capacity), unlike typical lithium-ion batteries. This lack of a voltage curve plateau means the output voltage being non-constant (varying too much) and would require a complicated voltage regulation algorithm to maintain the cell output voltage at a constant level.(2) Actually, the discharge curve for the Ni-ion cell exhibits an extremely sharp drop in voltage from 1.5 volts to below 0.6 volts as soon as the discharge process begins and, during most of the discharge process, the cell output is below 0.6 volts, which is not very useful. As a point of reference, the alkaline cell (a primary battery) provides an output voltage of 1.5 volts.(3) The discharge curves are characteristic of surface adsorption or electroplating mechanisms at the cathode, as opposed to ion intercalation. Further, it appears that the main event that occurs at the cathode during the battery discharge is electroplating. The high specific capacity values reported by Xu, et al. are simply a reflection on the high amount of Ni or Zn metal electroplated on the surfaces of graphene or CNTs. Since there is an excess amount of Ni or Zn in the anode, the amount of electroplated metal increases as the discharge time increases. Unfortunately, the electrochemical potential difference between the anode and the cathode continues to decrease since the difference in the metal amount between the anode and the cathode continues to decrease (more Zn or Ni is dissolved from the anode and gets electroplated on cathode surfaces). This is likely why the cell output voltage continues to decrease. The cell voltage output would be essentially zero when the amounts of metal at the two electrodes are substantially equivalent or identical. Another implication of this electroplating mechanism is the notion that the total amount of the metal that can be deposited on the massive surfaces at the cathode is dictated by the amount of the metal implemented at the anode when the cell is made. The high specific capacity (as high as 2,500 mAh/g) of graphene sheets at the cathode simply reflects the excessively high amount of Zn provided in the anode. There is no other reason or mechanism for why graphene or CNTs could “store” so much metal. The abnormally high specific capacity values as reported by Xu, et al. were artificially obtained based on the high amounts of Ni or Zn electroplated on cathode material surfaces, which unfortunately occurred at very low voltage values and are of little utility value.

Clearly, an urgent need exists for new cathode materials that provide proper discharge voltage profiles (having a high average voltage and/or a high plateau voltage during discharge), high specific capacity at both high and low charge/discharge rates (not just at a low rate), and long cycle-life for a multivalent metal secondary battery. Hopefully, the resulting battery can deliver some positive attributes of a supercapacitor (e.g. long cycle life and high power density) and some positive features of a lithium-ion battery (e.g. moderate energy density). These are the main objectives of the instant disclosure.

SUMMARY

The disclosure provides a multivalent metal-ion battery comprising an anode, a cathode, and an electrolyte in ionic contact with the anode and the cathode to support reversible deposition and dissolution of a multivalent metal (selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof) at the anode, wherein the anode contains the multivalent metal or its alloy as an anode active material and the cathode comprises a cathode active layer of exfoliated graphite or carbon material having inter-flake pores from 2 nm to 10 μm in pore size. The multivalent metal alloy preferably contains at least 80% by weight of the multivalent element in the alloy (more preferably at least 90% by weight). There is no restriction on the type of alloying elements that can be chosen.

The exfoliated carbon or graphite material in the cathode active layer may be selected from a thermally exfoliated product of meso-phase pitch, meso-phase carbon, meso carbon micro-beads (MCMB), coke particles, expanded graphite flakes, artificial graphite particles, natural graphite particles, highly oriented pyrolytic graphite, soft carbon particles, hard carbon particles, multi-walled carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, carbonized polymer fibers, or a combination thereof. These carbon/graphite materials can be subjected to an expansion treatment (e.g. intercalation, oxidation, and/or fluorination), followed by thermal exfoliation to obtain exfoliated graphite/carbon worms.

The exfoliated graphite/carbon worms are preferably recompressed to form a cathode active layer of recompressed exfoliated graphite or carbon material that is oriented in such a manner that the layer has a graphite edge plane in direct contact with the electrolyte (to readily admit ions from the electrolyte and release ions into electrolyte) and facing or touching the separator (so that the ions permeating through the porous separator can readily enter the inter-flake spaces near the edge plane).

In certain embodiments, the layer of recompressed exfoliated graphite or carbon material has a physical density from 0.5 to 1.8 g/cm3and has meso-scaled pores having a pore size from 2 nm to 50 nm. In some preferred embodiments, the layer of recompressed exfoliated graphite or carbon material has a physical density from 1.1 to 1.8 g/cm3and has pores having a pore size from 2 nm to 5 nm. In certain embodiments, the exfoliated graphite or carbon material has a specific surface area from 10 to 1,500 m2/g. The specific surface area of the recompressed worms is typically from 10 to 1,000 m2/g, but more typically and preferably from 20 to 300 m2/g.

We have observed that a select multivalent metal (e.g. Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Ga, In, or Cr), when coupled with a presently invented layer of exfoliated graphite or carbon material (preferably recompressed), can exhibit a discharge curve plateau at approximately 1.0 volt or higher (up to 3.7 volts). This plateau regime of a discharge voltage vs. time (or capacity) curve enables the battery cell to provide a useful constant voltage output. A voltage output significantly lower than 1 volt is generally considered undesirable. The specific capacity corresponding to this plateau regime is typically from approximately 100 mAh/g to above 600 mAh/g.

This multivalent metal-ion battery can further comprise an anode current collector supporting the aluminum metal or aluminum metal alloy or further comprise a cathode current collector supporting the cathode active layer. The current collector can be a mat, paper, fabric, foil, or foam that is composed of conducting nano-filaments, such as graphene sheets, carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, carbonized polymer fibers, or a combination thereof, which form a 3D network of electron-conducting pathways. The high surface areas of such an anode current collector not only facilitate fast and uniform dissolution and deposition of metal ions, but also act to reduce the exchange current density and, thus, the tendency to form metal dendrites that otherwise could cause internal shorting.

The carbon or graphite material, such as meso-phase pitch, meso-phase carbon, meso carbon micro-beads (MCMB), coke particles, expanded graphite flakes, artificial graphite particles, natural graphite particles, highly oriented pyrolytic graphite, soft carbon particles, hard carbon particles, multi-walled carbon nanotubes, carbon nano-fibers, carbon fibers, graphite nano-fibers, graphite fibers, and carbonized polymer fibers, has an original inter-planar spacing d002from 0.27 nm to 0.42 nm prior to a chemical or physical expansion treatment and the inter-planar spacing d002is increased to from 0.43 nm to 2.0 nm after the expansion treatment.

The expansion treatment includes an expansion treatment includes an oxidation, fluorination, bromination, chlorination, nitrogenation, intercalation, combined oxidation-intercalation, combined fluorination-intercalation, combined bromination-intercalation, combined chlorination-intercalation, or combined nitrogenation-intercalation of the graphite or carbon material. This expansion treatment is followed by thermal exfoliation and recompression. Due to the expansion treatments, the carbon or graphite material can contain a non-carbon element selected from oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or boron.

Unconstrained thermal exfoliation typically results in exfoliated graphite/carbon worms that have inter-flake pores having an average size from 20 nm to 50 μm (more typically from 100 nm to 10 μm). The exfoliated graphite/carbon worms are then compressed to produce a layer or block of recompressed exfoliated carbon or graphite material that is oriented in such a manner that the layer has a graphite edge plane in direct contact with the electrolyte and facing or contacting the separator. The recompressed exfoliated carbon or graphite material typically has a physical density from 0.5 to 1.8 g/cm3and has meso-scaled pores having a pore size from 2 nm to 50 nm.

The electrolyte may be selected from an aqueous electrolyte, organic electrolyte, polymer electrolyte, molten salt electrolyte, ionic liquid electrolyte, or a combination thereof. In the invented multivalent metal-ion battery, the electrolyte may contain NiSO4, ZnSO4, MgSO4, CaSO4, BaSO4, FeSO4, MnSO4, CoSO4, VSO4, TaSO4, CrSO4, CdSO4, GaSO4, Zr(SO4)2, Nb2(SO4)3, La2(SO4)3, BeCl2, BaCl2, MgCl2, AlCl3, Be(ClO4)2, Ca(ClO4)2, Mg(ClO4)2, Mg(BF4)2, Ca(BF4)2, Be(BF4)2, tri(3,5-dimethylphenyl borane, tris(pentafluorophenyl)borane, Alkyl Grignard reagents, magnesium dibutyldiphenyl, Mg(BPh2Bu2)2, magnesium tributylphenyl Mg(BPhBu3)2), or a combination thereof.

In certain embodiments of the present disclosure, the electrolyte comprises at least a metal ion salt selected from a transition metal sulfate, transition metal phosphate, transition metal nitrate, transition metal acetate, transition metal carboxylate, transition metal chloride, transition metal bromide, transition metal perchlorate, transition metal hexafluorophosphate, transition metal borofluoride, transition metal hexafluoroarsenide, or a combination thereof.

In certain embodiments, the electrolyte comprises at least a metal ion salt selected from a metal sulfate, phosphate, nitrate, acetate, carboxylate, chloride, bromide, or perchlorate of zinc, aluminum, titanium, magnesium, beryllium, calcium, manganese, cobalt, nickel, iron, vanadium, tantalum, gallium, chromium, cadmium, niobium, zirconium, lanthanum, or a combination thereof.

In the multivalent metal-ion battery, the electrolyte comprises an organic solvent selected from ethylene carbonate (EC), dimethyl carbonate (DMC), methylethyl carbonate (MEC), diethyl carbonate (DEC), methyl butyrate (MB), ethyl propionate, methyl propionate, propylene carbonate (PC), γ-butyrolactone (γ-BL), acetonitrile (AN), ethyl acetate (EA), propyl formate (PF), methyl formate (MF), tetrahydrofuran (THF), toluene, xylene, methyl acetate (MA), or a combination thereof.

In certain embodiments, the layer of exfoliated carbon or graphite material also operates as a cathode current collector to collect electrons during a discharge of the battery and wherein the battery contains no separate or additional cathode current collector.

The cathode active layer of exfoliated graphite/carbon may further comprise a binder material (preferably an electrically conductive binder material), which bonds exfoliated graphite flakes together to form a cathode electrode layer. The electrically conductive binder material may be selected from coal tar pitch, petroleum pitch, meso-phase pitch, a conducting polymer, a polymeric carbon, or a derivative thereof. Non-conducting materials (e.g. PVDF, PTFE, SBR, etc.) may also be used.

Typically, the invented secondary battery has an average discharge voltage typically no less than 1 volt (more typically and preferably >1.5 volts) and a cathode specific capacity greater than 200 mAh/g (preferably and more typically >300 mAh/g, more preferably >400 mAh/g, and most preferably >500 mAh/g) based on a total cathode active layer weight. Some cells deliver a specific capacity >600 mAh/g.

Preferably, the secondary battery has an average discharge voltage no less than 2.0 volts (preferably >2.5 volts, more preferably >3.0 volts, and most preferably >3.5 volts) and a cathode specific capacity greater than 100 mAh/g based on a total cathode active layer weight (preferably and more typically >200 mAh/g, more preferably >300 mAh/g, and most preferably >500 mAh/g).

The present disclosure also provides a method of manufacturing a multivalent metal-ion battery. The method comprises: (a) providing an anode containing a multivalent metal (selected from Ni, Zn, Be, Mg, Ca, Ba, La, Ti, Ta, Zr, Nb, Mn, V, Co, Fe, Cd, Cr, Ga, In, or a combination thereof) or its alloy; (b) providing a cathode comprising a layer of recompressed exfoliated carbon or graphite material (recompressed graphite/carbon worms or recompressed expanded graphite flakes, not graphene sheets); and (c) providing an electrolyte capable of supporting reversible deposition and dissolution of the multivalent metal at the anode and reversible adsorption/desorption and/or intercalation/de-intercalation of ions at the cathode; wherein the layer of recompressed exfoliated carbon or graphite material is oriented in such a manner that the layer has a graphite edge plane in direct contact with the electrolyte and facing or contacting a plane of the separator. Typically, this graphite edge plane is substantially parallel to the porous separator and, thus, can readily admit and accommodate ions that migrate through the separator. Preferably, the electrolyte contains an aqueous electrolyte, an organic electrolyte, a molten salt electrolyte, or an ionic liquid.

The method can further include providing a porous network of electrically conductive nano-filaments to support the multivalent metal or its alloy at the anode.

In the method, the step of providing a cathode preferably contains subjecting a carbon or graphite material to an expansion treatment selected from an oxidation, fluorination, bromination, chlorination, nitrogenation, intercalation, combined oxidation-intercalation, combined fluorination-intercalation, combined bromination-intercalation, combined chlorination-intercalation, or combined nitrogenation-intercalation, followed by thermal exfoliation at a temperature from 100° C. to 2,500° C.

In certain preferred embodiments, the procedure of providing the cathode includes compressing exfoliated graphite or carbon using a wet compression or dry compression to align constituent graphite flakes of the exfoliated graphite or carbon. The procedure can produce a layer or block of graphitic structure having a flake edge plane being parallel to the separator, enabling direct entry of ions from separator pores into inter-flake spaces with minimal resistance. Preferably, the procedure of providing the cathode includes compressing exfoliated graphite or carbon using a wet compression to align constituent graphite flakes of the exfoliated graphite or carbon, wherein wet compression includes compressing or pressing a suspension of exfoliated graphite or carbon dispersed in a liquid electrolyte intended for use in a multivalent metal-ion cell. Any of the aforementioned electrolytes can be utilized in this suspension. The electrolyte later becomes part of the electrolyte of the intended multivalent metal-ion battery.

