DESC:FORM 2
The Patents Act 1970
(39 of 1970)
&
The Patent Rules 2003
COMPLETE SPECIFICATION
(See Section 10 and rule 13)
TITLE OF THE INVENTION:
“GRAPHENE-INCORPORATED POSITIVE ELECTRODE MATERIAL FOR ULTRA-FAST-CHARGING LITHIUM-ION BATTERY”
APPLICANT:
Name: Cappatery Pvt Ltd
Nationality: Indian
Address: L3/108, Acharya Vihar, Bhubaneswar- 751013
PREAMBLE OF THE DESCRIPTION:
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED
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A) TECHNICAL FIELD OF INVENTION
[001] The present invention generally relates to a process for the incorporation of graphene as an additive in the preparation of the cathode material for lithium-ion battery, which is electrochemically stable and can accommodate ultra-fast charge transfer kinetics during charging and discharging. The said battery contains an anode, a membrane separator and a cathode, arranged in a pouch/prismatic cell configuration. The anode is made of standard graphite-based or graphite-silicone based material. The ion-channelling system comprises an electrolyte that fills the free space between the anode and cathode. The cathode is made up of a proportionate mixture of graphene and NMC-811 in appropriate weight ratio.
B) BACKGROUND OF INVENTION
[002] The development of energy storage devices in general has gained significance to meet the energy demands of the modern life. Lithium-ion systems have seen phenomenal progress in this context owing primarily to their high energy per weight ratio. Nevertheless, the limited availability and several other geopolitical issues are a matter of grave concern which surrounds the future of the lithium-based storage devices. Therefore, minimal utilisation of lithium and recycling of the existing lithium-based devices are probably the two best options to tackle such issues.
[003] The rapidly increasing environmental hazards throughout the globe have a major contribution from the combustion of fossil fuels. Since few years, the situation has been aggravating at an alarming rate. Although significant amount of concern has been expressed by a vast number of countries and global policy-makers, the issue remains stand-still, as the coordination between research laboratories and industries is facing severe lack of interest. The only way these environmental hazards can be tackled is by moving towards clean, green, and sustainable energy resources. And, this could be achieved by a large-scale
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implementation of renewable energy resources. The solar power is the foremost amongst all the probable renewable energy solution, and is gaining a steady growth for the past few years. Besides solar power, wind energy, hydroelectricity, and geothermal energy can also be considered to harvest clean energy and promote sustainable growth. The advantage with the renewable energy resources is that the energy is harvested in the form of electricity, which acts as an excellent energy carrier and can be directly implemented in vast range of applications. However, the only issue with these renewable energy resources is that they require plenty of storage options for the uniform distribution and/or transmission of the harvested energy, as these resources are intermittent/volatile in nature and are mostly affected by the surrounding environments and/or climatic conditions. Since the trade-off is between energy and cost, the storage devices are expected to be effective in the long run, and should be affordable as well as portable. Currently, Li-ion technology is the only commercially viable storage solution that can offer excellent portability. Unlike other secondary batteries, Li-ion batteries are lightweight and are ideal for smart electronic devices that need space management and portability. [004] In the recent days, the lithium-ion batteries use a solid reductant as the anode and a solid oxidant as the cathode. On discharge, the metallic anode supplies Li+ ions to the electrolyte and electrons to the external circuit. The cathode is typically an electronically conducting host into which Li+ ions are inserted reversibly from the electrolyte as a guest species and charge-compensated by electrons from the external circuit. The chemical reactions at the anode and cathode of a lithium secondary battery must be reversible. On charge, removal of electrons from the cathode by an external field releases Li+ ions back to the electrolyte to restore the parent host structure, and the addition of electrons to the anode by the external field attracts charge-compensating Li+ ions back into the anode to restore it to its original composition
[005] Due to extremely poor electrical conductivity of all cathode active materials in a lithium-ion or lithium metal cell, a conductive additive, typically in the amount of 2 – 15%, must be added into the electrode. However, the
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conventional conductive additives (graphite powder, conductive carbon black etc.) are not an electrode active material. The use of a non-active material means that the relative proportion of an electrode active material e.g., NMC-811, is reduced or diluted. For instance, the incorporation of 5% by weight of PVDF as a binder and 5% of carbon black as a conductive additive in a cathode would mean that the maximum amount of the cathode active mate rial e.g., NMC-811, is only 90%, effectively reducing the total lithium-ion storage capacity. Since the specific capacities of the more commonly used cathode active materials are already very low (140-170 mAh/g), this problem is further aggravated if a significant amount of non-active materials is used to dilute the concentration of the active material. [006] Typically, several drawbacks for carbon black (CB) materials, as a conductive additive: CBs are typically available in the form of aggregates of multiple primary particles that are typically spherical in shape. Due to this geometric feature and the notion that CBs are a minority phase dispersed as discrete particles in an electrically insulating matrix e.g., NMC-811, a large amount of CB is required to reach a percolation threshold where the CB particles are combined to form a 3D network of electron-conducting paths.
[007] CBs themselves possess low electrical conductivity and, hence, the resulting electrode too has a low conductivity, even when the percolation threshold is reached. A relatively high proportion of CB must be incorporated in the cathode to make the resulting composite electrode reasonably conducting.
[008] Clearly, an urgent need exists for a more effective electrically conductive additive material. Preferably, this electrically conductive additive should also have high thermal conductivity. Such a thermally conductive additive would be capable of dissipating the heat generated from the electrochemical operation of the Li-ion battery, thereby increasing the reliability of the battery and decreasing the likelihood that the battery will suffer from thermal runaway and rupture. With a high electrical conductivity, there would be no need to add a high proportion of conductive additives.
[009] There have been several attempts to use other carbon nano-materials than carbon black (CB) or acetylene black (AB) as a conductive additive for the
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cathode of a lithium battery. These include carbon nano-tubes (CNTs), vapor-grown carbon nano-fibers (VG-CNFs), and simple carbon coating on the Surface of cathode active material particles. The results, in either of the above cases, have not been satisfactory and hence, as of today, carbon black and artificial graphite particles are practically the only two types of cathode conductive additives widely used in lithium-ion battery industry. [0010] Hence, the reasons are beyond just the obvious high costs of both CNTs and VG-CNFs. The difficulty in disentangling CNTs and VG-CNFs and uniformly dispersing them in a liquid and/or solid media has been an impediment to the more widespread utilization of these expensive materials as a conductive additive. For the less expensive carbon coating, being considered for use in lithium iron phosphate, the conductivity of the carbon coating (typically obtained by converting a precursor such as sugar or resin via pyrolyzation) is relatively low. It would take a graphitization treatment to render the carbon coating more conductive, but this treatment requires a temperature in excess of 2000 °C., which would degrade the rest of the cathode components e.g., NMC-811.
[0011] In a similar approach, to find an alternate method, Ding et. al., investigated the electrochemical behavior of LiFePO4/graphene composites [Y. Ding, et al., Electrochemistry Communications, 12 (2010), 10-13]. The co-precipitation method leads to the formation of LiFePO4 nano-particles coated on the Surfaces of graphene nano sheets. The cathode is then prepared by stacking these LiFePO4-coated graphene sheets together. However, this approach has several major drawbacks: With the two primary surfaces of a graphene sheet heavily loaded with LiFePO4 nano-particles, the resulting electrode entails many insulator-to-insulator contacts between two adjoining coated sheets in a stack. With both LiFePO4 particles and graphene sheets being nano-scaled the coated sheets are also nano-scaled, making the preparation of electrodes very difficult. It is found that the nano particle-coated nano-graphene sheets, prepared by the co-precipitation method, are not amenable to fabrication of cathodes with the same equipment as well. In particular, these coated nano-graphene sheets could not be
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compacted into a dense state with a highmass per unit electrode volume. In other words, the cathode tap density is relatively low. [0012] Another prior art is US 2011/02873.16 A1 related to carbon nano-tube composites and particularly to carbon nano-tube compositions for electrochemical energy storage devices and a method for making the same.
[0013] Another prior art US 9,203,084 B2 relates to a cathode (positive electrode) of a lithium battery and a process for producing this cathode. The electrode comprises a cathode active material-coated graphene sheet and the graphene sheet has two opposed parallel surfaces, wherein at least 50% area (preferably greater than 80%) of one of the two surfaces is coated with a cathode active material coating. The graphene material is in an amount of from 0.1% to 99.5% by weight and the cathode active material is in an amount of at least 0.5% by weight (preferably greater than 80% and more preferably greater than 90%), all based on the total weight of the graphene material and the cathode active material combined. The cathode active material is preferably an inorganic material, an organic or polymeric material, a metal oxide/phosphate/sulphide, or a combination thereof. Also provided is a lithium battery, including a lithium-ion, lithium-metal, or lithium-sulphur battery.
[0014] Another prior art US 8,691.441 B2 relates to a nano graphene-enhanced particulate for use as a lithium battery cathode active material, wherein the particulate is formed of a single or a plurality of graphene sheets and a plurality of fine cathode active material particles with a size smaller than 10 µm (preferably sub-micron or nano-scaled), and the graphene sheets and the particles are mutually bonded or agglomerated into an individual discrete particulate with at least a graphene sheet embracing the cathode active material particles, and wherein the particulate has an electrical conductivity no less than 10-4 S/cm and the graphene is in an amount of from 0.01% to 30% by weight based on the total weight of graphene and the cathode active material combined.
[0015] Hence it is evident that despite rapid developments in cell chemistry and cell production process, the current battery cells have low energy density and cell chemistries also fails to handle rapid charging methods for longer period of time.
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[0016] In view of the above, the present invention provides a cell chemistry structure that allows a rapid charging without affecting the cell chemistry or output integrity.
[0017] The value additions and above-mentioned shortcomings, disadvantages and problems are addressed herein, as detailed below.
C) OBJECT OF INVENTION
[0018] The primary objective of the present invention is to facilitate multiple cathode active materials to aggregate into the nanographene-enhanced particulates.
[0019] Another objective of the present invention is to provide the nanographene-enhanced particulates that are more conducive formation of a 3D network of electron-conducting paths.
[0020] Another objective of the present invention is to provide the nanographene-enhanced particulates that are imparting the exceptional conductivity to the cathode and enabling the cathode to become high rate.
[0021] Yet another objective of the present invention is to propose the manufacturing of the aforesaid hybrid energy storage devices with improved performance levels and to reduce the manufacturing cost.
[0022] These and other objects and advantages of the embodiments herein will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
D) SUMMARY OF INVENTION
[0023] The various embodiments of the present invention relate to a graphene incorporated cathode material for lithium-ion battery, which is electrochemically stable and can accommodate ultra-fast charge transfer kinetics during charging and discharging. The battery contains an anode, a membrane separator and a cathode, arranged in a pouch/prismatic cell configuration. The anode is made of standard graphite-based or graphite-silicone based material. The cathode is made up of a proportionate mixture of graphene and NMC-811 in appropriate weight
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ratio. The lithium-ion battery comprises a nanographene-enhanced particulate which is formed of a plurality of graphene sheets and a plurality of fine cathode active material particles (primary particles) with a size smaller than 10µm (preferably smaller than 1 µm and further preferably <100 nm). The plurality of graphene sheets and the primary particles are mutually bonded or agglomerated into the nanographene-enhanced particulate (also referred to as a secondary particle) with at least a graphene sheet embracing the cathode active material particles. Some of the graphene sheets are get incorporated into an interior of the particulate and providing additional electron-conducting paths. [0024] Typically, the resulting particulate has an electrical conductivity no less than 10-2 S/cm (typically and preferably greater than 10-2 S/cm). The graphene component is preferably between 0.5% and 10% based on the total weight of graphene and the cathode active material combined. With the processes herein invented and reported, the particulates tend to be approximately spherical in shape, which is a desirable feature.
[0025] In another embodiment, a process for preparing a nanographene-enhanced particulate, comprising the steps of: (a) preparing a precursor mixture of graphene or graphene precursor with a cathode active material or a precursor to the active material; (b) thermally and/or chemically converting the precursor mixture to the graphene-enhanced cathode particulate; (c) dispersing or immersing a laminar graphite material (e.g., graphite powder) in a mixture of an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or graphite oxide (GO); (d) exposing the resultant GIC or GO to a thermal shock, preferably within a temperature range of 600 – 1,100° C, for a short period of time (typically 15 to 60 seconds), to obtain exfoliated graphite or graphite worms; and (e) dispersing exfoliated graphite in a liquid (e.g., water) and mechanically separating individual nano graphene platelets or sheets from graphite worms using, for instance, a high-shear mixer or an ultrasonicator to obtain a graphene or graphene precursor suspension; or, alternatively.
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[0026] Further, the process may involve (f) re-dispersing the exfoliated graphite to a liquid medium containing an acid (e.g., sulfuric acid), an oxidizing agent (e.g., nitric acid), or an organic solvent (e.g., NMP) at a desired temperature for a duration of time until the exfoliated graphite is converted into graphene oxide or graphene dissolved in the liquid medium. The acid is preferably a weak acid (such as diluted sulfuric acid) or a more environmentally benign acid, such as formic acid, acetic acid, citric acid, carboxylic acid, and/or combinations thereof.
[0027] These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.
E) BRIEF DESCRIPTION OF DRAWINGS
[0028] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
[0029] Figure 1 shows the schematic representation of the working mechanism of a standard Li-ion system in which the lithium ions move inside the cell which are balanced by the flow of equal number of electrons in the external circuit.
[0030] It is to be noted that the amount of charge (lithium ion and/or electrons) flow is limited by the nature of the electrode material(s), i.e., the flow is largely dependent on the conductivity of the electrode. Higher the conductivity of the electrode, better will be the charge transfer rate.
[0031] Figure 2 shows the schematic illustration of the working mechanism of the graphene battery, where the graphene sheets are uniformly disposed on the exterior surface of the cathode particulates, naturally forming a 3-D network of
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electron-conducting paths when these particulates are packed together to form the final electrode. [0032] The rate/number of charges transferred in this case is higher than what is observed in the case of a standard Li-ion system.
F) DETAILED DESCRIPTION OF DRAWINGS
[0033] In the following detailed description, a reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. The embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical, electronic and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.
[0034] This invention provides a nanographene-enhanced particulate for use as a lithium battery cathode active material. As illustrated in Figure 2, the particulate is formed of a single or a plurality of graphene sheets and a plurality of fine cathode active material particles (primary particles) with a size Smaller than 10 µm (preferably smaller than 1 µm and further preferably <100 nm). The graphene sheets and the primary particles are mutually bonded or agglomerated into the particulate (also referred to as a secondary particle) with at least a graphene sheet embracing the cathode active material particles. Some graphene sheets get incorporated into the interior of the particulate, providing additional electron-conducting paths.
[0035] Another preferred embodiment, wherein an additional conductive additive (such as carbon black particles and/or carbon coating) is incorporated in the particulate prior to mixing with graphene. The resulting particulate typically has an electrical conductivity no less than 10 S/cm (typically and preferably greater than 10 S/cm). The graphene component is preferably between 0.5% and 10%, based on the total weight of graphene and the cathode active material combined. With the processes herein invented and reported, the particulates tend to be
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approximately spherical in shape, which is a desirable feature. A nanographene platelet (NGP) or nanographene sheet is composed of one basal plane (graphene plane) or multiple basal planes stacked together along the c-axis. In a graphene plane, carbon atoms occupy a 2-D hexagonal lattice in which carbon atoms are bonded together through strong in-plane covalent bonds. Along the c-axis or thickness direction, these graphene planes may be weakly bonded together through van der Waals forces. An NGP can have a platelet thickness from less than 0.34 nm (single layer) to 100 nm (multi-layer). For the present electrode use, the preferred thickness is <10 nm and most preferably <3 nm or 10 layers, since more than 10 layers would turn the system close to graphite rather than graphene. The presently invented graphene-enhanced particulate preferably contains mostly single-layer graphene, but could make use of some few-layer graphene (less than 10 layers). The graphene sheet may contain a small amount (typically <20% by weight) of non-carbon elements, such as hydrogen, fluorine, and oxygen, which are attached to the edge or surface of the graphene plane. The high thermal conductivity of graphene (~5,000 W/mK) is clearly an advantageous property that could not be achieved by any other type of conductive additives. Graphene sheets may be oxidized to various extents during their preparation, resulting in graphite oxide (GO) or graphene oxide. Hence, in the present context, graphene preferably or primarily refers to those graphene sheets containing no or low oxygen content. As a preferred embodiment, the process of producing graphene-enhanced particulates comprises (i) preparing a precursor mixture of graphene or graphene precursor with a cathode active material or a precursor to the active material; [0036] and (ii) thermally and/or chemically converting the precursor mixture to the graphene-enhanced cathode particulate. Described in more detail, the process entails:
[0037] (a) dispersing or immersing a laminar graphite material (e.g., graphite powder) in a mixture of an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or graphite oxide (GO);
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[0038] (b) exposing the resultant GIC or GO to a thermal shock, preferably within a temperature range of 600 – 1,100 ?, for a short period of time (typically 15 to 60 seconds), to obtain exfoliated graphite or graphite worms; and
[0039] (c) dispersing exfoliated graphite in a liquid (e.g., water) and mechanically separating individual nano graphene platelets or sheets from graphite worms using, for instance, a high-shear mixer or an ultrasonicator to obtain a graphene or graphene precursor suspension; or, alternatively,
[0040] (d) re-dispersing the exfoliated graphite to a liquid medium containing an acid (e.g., sulfuric acid), an oxidizing agent (e.g., nitric acid), or an organic solvent (e.g., NMP) at a desired temperature for a duration of time until the exfoliated graphite is converted into graphene oxide or graphene dissolved in the liquid medium. The acid is preferably a weak acid (such as diluted sulfuric acid) or a more environmentally benign acid, such as formic acid, acetic acid, citric acid, carboxylic acid, and/or combinations thereof. The exfoliated graphite, when dispersed in these acids, is gradually dispersed and essentially dissolved to form a graphene or graphene oxide solution or suspension. Although not a required operation, stirring, mechanical shearing, or ultrasonication can be used to accelerate the dispersion and dissolution step;
[0041] (e) dispersing a cathode active material or a precursor to a cathode active material to the graphene or graphene precursor solution or suspension prepared in step (c) or step (d) to obtain a precursor mixture suspension; and
[0042] (f) thermally and/or chemically converting the precursor mixture to the graphene-enhanced cathode particulate. An optional, but desirable intermediate step between (e) and (f) involves drying the suspension to form the precursor mixture in a solid state. If the precursor mixture contains a precursor to a cathode active material, the mixture will be subject to thermal treatment (sintering) to obtain the particulates that contain primary cathode particles therein (e.g., at 700 ?.). If the precursor mixture contains a precursor to graphene (e.g., graphene oxide), then the precursor may be subjected to either chemical or thermal oxidation. A heat treatment at a temperature of preferably 500-1,000 ? for 1-2
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hours would serve to eliminate a majority of the oxygen content from the graphene sheets. [0043] In step (e), carbon black particles may be added along with the cathode active material particles. Alternatively, the cathode active material particles may be coated with a thin layer of carbon before they are mixed with the graphene suspension. These coated particles are then heat-treated at a temperature of 500-1,000 ?. to obtain carbon-coated particles. These particles are then added to the graphene solution or suspension.
[0044] As another preferred embodiment, the process may begin with the production of a precursor solution or suspension of pristine graphene (non-oxidized graphene) directly from graphite particles, which is followed by the addition of a cathode active material or precursor to the cathode active material to this solution or suspension to obtain a precursor mixture. The production of a precursor solution or suspension may include the following steps:
[0045] (a) Preparing a suspension containing pristine nano graphene platelets (NGPs) dispersed in a liquid medium using, for instance, direct ultrasonication (e.g., a process disclosed by us in U.S. patent application Ser. No. 1 1/800,728 (May 8, 2007));
[0046] (b) Optionally removing some of the liquid from the suspension; 0051 (c) Adding a desired amount of a cathode active material or a precursor to a cathode active material to obtain a precursor mixture suspension or solution;
[0047] (d) Removing the liquid from the suspension to obtain a precursor mixture solid; and
[0048] (e) Thermally and/or chemically converting the precursor mixture solid to the graphene-enhanced cathode particulate.
[0049] For the preparation of a cathode, multiple graphene enhanced particulates are mixed with a binder solution (e.g., PVDF in NMP) to obtain a slurry or paste. A desired amount of the slurry or paste is then coated onto a current collector, allowing the liquid to evaporate and leaving behind an electrode bonded to a surface of a current electrode. For examples, lithium cobalt oxide particles embraced by graphene sheets may be added to a solution containing a solvent
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(NMP). The resulting paste may be coated onto an aluminum foil as a current collector to form a coating layer of 50-500 µm thick. [0050] In the aforementioned examples, the starting material for the preparation of 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.
[0051] 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 ?.) for a sufficient length of time (typically 30 minutes to 5 days). 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) to obtain graphite intercalation compound (GIC). The GIC particles are then exposed to a thermal treatment, preferably in a temperature range of 600-1,100 ?. 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. 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 polymer solution or suspension.
[0052] 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
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containing a surfactant or dispersing agent to obtain a suspension and Subjecting the suspension to direct ultrasonication. [0053] The presently invented process typically resulted in nanographene sheets that, when formed into a thin film with a thickness no greater than 100 nm, exhibits an electrical conductivity of at least 10 S/cm, often higher than 100 S/cm, and, in many cases, higher than 1,000 S/cm. The resulting NGP powder material typically has a specific surface area of from approximately 300 m/g to 2,600 m/g and, in many cases, comprises single-layer graphene or few-layer graphene sheets.
[0054] When these graphene sheets are combined with cathode active material particles to form graphene-enhanced particulates, these particulates (when packed into a dry electrode) exhibit an electrical conductivity no less than 10 S/cm (typically and preferably greater than 10 S/cm). The graphene component is in an amount of from 0.01% to 30% by weight (preferably between 0.1% to 20% by weight and more preferably between 0.5% and 10%) based on the total weight of graphene and the cathode active material combined. Preferably, the particulates are approximately spherical in shape.
[0055] Preferably, the cathode active material is selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium Vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium Vanadium phosphate, lithium mixed metal phosphates, metal sulfides, and combinations thereof. The cathode active material particles (primary particles) in the particulate (secondary particle) preferably have a dimension Smaller than 1 um and further preferably smaller than 100 nm. Smaller dimensions promote shorter lithium diffusion times and faster battery charge and discharge rates.
[0056] A particularly desirable group of cathode active materials consists of lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphates, and combinations thereof, wherein the cathode active material contains Sub-micron or nano-scaled particles with a size less than 1 µm, preferably <100 nm. This class of cathode active materials is
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relatively safe and is a preferred class of cathode active materials in the lithium ion batteries for electric vehicle applications. [0057] Optionally, the particulate further comprises a carbon material in electronic contact with the cathode active material and a graphene sheet. This carbon material can be a polymeric carbon, amorphous carbon, chemical vapor deposition (CVD) carbon, carbon black (CB), acetylene black (AB), activated carbon, fine expanded graphite particle with a dimension smaller than 100 nm, artificial graphite particle, and/or natural graphite particle. Preferably, the carbon material is coated on at least one of the cathode active material particles and more preferably on the surface of all primary particles, which are than embraced by a graphene sheet or a plurality of graphene sheets.
[0058] According to the embodiments of the present invention relates to a nanographene-enhanced particulate for use as a lithium battery cathode active material.
[0059] As illustrated in Figure 1 is a schematic representation of working mechanism of a standard lithium-ion system in which the lithium ions move inside the cell which is balanced by the flow of an equal number of electrons in the external circuit. It is to be noted that the amount of charge (lithium-ion and/or electrons) flow is limited by the nature of the electrode material(s), i.e., the flow is largely dependent on the conductivity of the electrode. The nanographene-enhanced particulate is formed of a plurality of graphene sheets and a plurality of fine cathode active material particles (primary particles) with a size smaller than 10µm (preferably smaller than 1 µm and further preferably <100 nm). The plurality of graphene sheets and the primary particles are mutually bonded or agglomerated into the nanographene-enhanced particulate (also referred to as a secondary particle) with at least a graphene sheet embracing the cathode active material particles. Some of the graphene sheets are get incorporated into an interior of the particulate and providing additional electron-conducting paths.
[0060] Another preferred embodiment, an additional conductive additive (such as carbon black particles and/or carbon coating) is incorporated in the nanographene-enhanced particulate prior to mixing with graphene material. The resulting
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particulate typically has an electrical conductivity no less than 10-2 S/cm (typically and preferably greater than 10-2 S/cm). The graphene component is preferably between 0.5% and 10% based on the total weight of graphene and the cathode active material combined. With the processes herein invented and reported, the particulates tend to be approximately spherical in shape, which is a desirable feature. A nanographene platelet (NGP) or nanographene sheet is composed of one basal plane (graphene plane) or multiple basal planes stacked together along a c-axis. [0061] Accordingly, the graphene amount is from 0.1% to 20% by weight of the total weight of graphene and the cathode active material combined. The graphene amount is from 0.5% to 10% by weight of the total weight of graphene and the cathode active material combined.
[0062] In a graphene plane, carbon atoms occupy a 2-D hexagonal lattice in which carbon atoms are bonded together through strong in-plane covalent bonds. Along the c-axis or thickness direction, these graphene planes are weakly bonded together through van der Waals forces. The NGP can have a platelet thickness from less than 0.34 nm (single layer) to 100 nm (multi-layer). For the present electrode use, the preferred thickness is <10 nm and most preferably <3 nm or 10 layers, since more than 10 layers would turn the system close to graphite rather than graphene. The presently invented graphene-enhanced particulate preferably contains mostly single-layer graphene, but could make use of some few-layer graphene (less than 10 layers). The graphene sheets may contain a small amount (typically <20% by weight) of non-carbon elements, such as hydrogen, fluorine, and oxygen, which are attached to the edge or surface of the graphene plane.
[0063] Accordingly, the high thermal conductivity of graphene (~5,000 W/mK) is clearly an advantageous property that could not be achieved by any other type of conductive additives. The graphene sheets are oxidized to various extents during their preparation, resulting in graphite oxide (GO) or graphene oxide. Hence, in the present invention, the graphene preferably or primarily refers to those graphene sheets containing no or low oxygen content.
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[0064] As a preferred embodiment, a process for preparing a nanographene-enhanced particulate, comprising the steps of: (a) preparing a precursor mixture of graphene or graphene precursor with a cathode active material or a precursor to the active material; and (b) thermally and/or chemically converting the precursor mixture to the graphene-enhanced cathode particulate.
[0065] Further, the process may involve (c) dispersing or immersing a laminar graphite material (e.g., graphite powder) in a mixture of an intercalant and an oxidant (e.g., concentrated sulfuric acid and nitric acid, respectively) to obtain a graphite intercalation compound (GIC) or graphite oxide (GO); (d) exposing the resultant GIC or GO to a thermal shock, preferably within a temperature range of 600 – 1,100° C, for a short period of time (typically 15 to 60 seconds), to obtain exfoliated graphite or graphite worms; and (e) dispersing exfoliated graphite in a liquid (e.g., water) and mechanically separating individual nano graphene platelets or sheets from graphite worms using, for instance, a high-shear mixer or an ultrasonicator to obtain a graphene or graphene precursor suspension; or, alternatively.
[0066] Further, the process may involve (f) re-dispersing the exfoliated graphite to a liquid medium containing an acid (e.g., sulfuric acid), an oxidizing agent (e.g., nitric acid), or an organic solvent (e.g., NMP) at a desired temperature for a duration of time until the exfoliated graphite is converted into graphene oxide or graphene dissolved in the liquid medium. The acid is preferably a weak acid (such as diluted sulfuric acid) or a more environmentally benign acid, such as formic acid, acetic acid, citric acid, carboxylic acid, and/or combinations thereof. The exfoliated graphite, when dispersed in these acids, is gradually dispersed and essentially dissolved to form a graphene or graphene oxide solution or suspension. Although not a required operation, stirring, mechanical shearing, or ultrasonication can be used to accelerate the dispersion and dissolution step.
[0067] Further, the process may involve dispersing a cathode active material or a precursor to a cathode active material to the graphene or graphene precursor solution or suspension prepared in the above-mentioned steps (c) and (d) to obtain
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a precursor mixture suspension; and thermally and/or chemically converting the precursor mixture to the graphene-enhanced cathode particulate. [0068] An optional, but desirable intermediate steps between (e) and (f) involves drying the suspension to form the precursor mixture in a solid state. If the precursor mixture contains a precursor to a cathode active material, the mixture will be subject to thermal treatment (sintering) to obtain the particulates that contain primary cathode particles therein (e.g., at 700). If the precursor mixture contains a precursor to graphene (e.g., graphene oxide), then the precursor may be subjected to either chemical or thermal oxidation. A heat treatment at a temperature of preferably 500-10000C for 1-2 hours would serve to eliminate a majority of the oxygen content from the graphene sheets.
[0069] In the step (e), the carbon black particles are added along with the cathode active material particles. Alternatively, the cathode active material particles are coated with a thin layer of carbon before they are mixed with the graphene suspension. The coated particles are heat-treated at a temperature of 500-1000 0C to obtain the carbon-coated particles. The particles are then added to the graphene solution or suspension. The graphene in the particulate has oxygen in the range of 5% to 25% by weight.
[0070] Accordingly, the lithium battery cathode comprising multiple nano graphene-enhanced cathode particulates such that the multiple particulates are packed together with graphene sheets forming a three-dimensional electron-conducting pathway. The multiple particulates are packed together with graphene sheets and said carbon material together forming a three-dimensional electron-conducting pathway. The lithium battery comprising an anode, a cathode, and a separator disposed between the anode and the cathode. Further, the lithium battery comprises an electrolyte in physical contact with both the anode and the cathode.
[0071] As another preferred embodiment, the process may begin with the production of a precursor solution or suspension of pristine graphene (non-oxidized graphene) directly from graphite particles, which is followed by the addition of a cathode active material or precursor to the cathode active material to this solution or suspension to obtain a precursor mixture.
20
[0072] Further, the production of a precursor solution or suspension may include the following steps: (a) preparing a suspension containing pristine nano graphene platelets (NGPs) dispersed in a liquid medium using, for instance, direct ultrasonication (e.g., a process disclosed by us in U.S. patent application Ser. No. 1 1/800,728 (May 8, 2007)); (b) optionally removing some of the liquid from the suspension; (c) Adding a desired amount of a cathode active material or a precursor to a cathode active material to obtain a precursor mixture suspension or solution; (d) removing the liquid from the suspension to obtain a precursor mixture solid; and (e) thermally and/or chemically converting the precursor mixture solid to the graphene-enhanced cathode particulate.
[0073] For the preparation of a cathode, multiple graphene enhanced particulates are mixed with a binder solution (e.g., PVDF in NMP) to obtain a slurry or paste. A desired amount of the slurry or paste is then coated onto a current collector, allowing the liquid to evaporate and leaving behind an electrode bonded to a surface of a current electrode. For examples, lithium cobalt oxide particles embraced by graphene sheets may be added to a solution containing a solvent (NMP). The resulting paste may be coated onto an aluminium foil as a current collector to form a coating layer of 50-500 µm thick.
[0074] In the examples, the starting material for the preparation of 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.
[0075] The graphite oxide is 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 30 minutes to 5 days). In order to reduce the time required to produce a precursor solution or suspension, one may choose
21
to oxidize the graphite to some extent for a shorter period of time (e.g., 30 minutes) to obtain graphite intercalation compound (GIC). The GIC particles are then exposed to a thermal treatment, preferably in a temperature range of 600-1100 °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. 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 polymer solution or suspension. [0076] 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.
[0077] In accordance with the present invention related to a process typically resulted in nanographene sheets that, when formed into a thin film with a thickness no greater than 100 nm, exhibits an electrical conductivity of at least 10 S/cm, often higher than 100 S/cm, and, in many cases, higher than 1,000 S/cm. The resulting NGP powder material typically has a specific surface area of from approximately 300 m/g to 2,600 m/g and, in many cases, comprises single-layer graphene or few-layer graphene sheets.
[0078] When the graphene sheets are combined with cathode active material particles to form graphene-enhanced particulates, these particulates (when packed into a dry electrode) exhibit an electrical conductivity no less than 10 S/cm (typically and preferably greater than 10 S/cm). The graphene component is in an amount of from 0.01% to 30% by weight (preferably between 0.1% to 20% by weight and more preferably between 0.5% and 10%) based on the total weight of
22
graphene and the cathode active material combined. Preferably, the particulates are approximately spherical in shape. [0079] Preferably, the cathode active material is selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium Vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium Vanadium phosphate, and lithium mixed metal phosphates, metal sulphides, and combinations thereof. The cathode active material particles (primary particles) in the particulate (secondary particle) preferably have a dimension Smaller than 1 um and further preferably smaller than 100 nm. Smaller dimensions promote shorter lithium diffusion times and faster battery charge and discharge rates.
[0080] Particularly desirable group of cathode active materials consists of lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphates, and combinations thereof, wherein the cathode active material contains Sub-micron or nano-scaled particles with a size less than 1 µm, preferably <100 nm. This class of cathode active materials is relatively safe and is a preferred class of cathode active materials in the lithium-ion batteries for electric vehicle applications.
[0081] Optionally, the particulate further comprises a carbon material in electronic contact with the cathode active material and a graphene sheet. This carbon material can be a polymeric carbon, amorphous carbon, chemical vapor deposition (CVD) carbon, carbon black (CB), acetylene black (AB), activated carbon, fine expanded graphite particle with a dimension smaller than 100 nm, artificial graphite particle, and/or natural graphite particle. Preferably, the carbon material is coated on at least one of the cathode active material particles and more preferably on the surface of all primary particles, which are than embraced by a graphene sheet or a plurality of graphene sheets.
[0082] The following examples serve to provide the best modes of practice for the present invention and should not be construed as limiting the scope of the invention. The various embodiments of the present invention are generally directed to a hybrid lithium-ion battery with supercapacitor membrane and
23
graphene section for allowing rapid charging without cell chemistry distortion or heating of cells.
[0083] In another embodiment, the lithium-ion battery includes a cathode which contains
nano graphene-enhanced particulates. The nano graphene-enhanced particulates are formed with a single sheet of graphene or a plurality of graphene sheets and a plurality of fine cathode active material particles. The one or more graphene sheets and the plurality of cathode active material particles are mutually bonded or agglomerated into the nanographene-enhanced particulates. Some of the graphene sheets are incorporated into an interior of the nanographene-enhanced particulate and providing additional electron-conducting paths.
[0084] The graphene sheets are oxidized to various extents during preparation of the one or more graphene sheets. Hence, the graphene preferably or primarily refers to the one or more graphene sheets which contain low oxygen content. Further, the graphene is fluorinated to a controlled extent to obtain graphite fluoride. The one or more graphene sheets contain a small amount (typically <25% by weight) of non-carbon elements such as hydrogen, fluorine, and oxygen, which are attached to an edge or surface of graphene planes.
[0085] In another embodiment, the lithium-ion battery may include a carbon material which is in an electronic contact with the cathode active materials and the one or more graphene sheets. The amount of graphene is at least 0.01% by weight and the amount of the cathode active material is at least 0.1% by weight based on the total weight of the particulate.
[0086] In another embodiment, the cathode active material particles are selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium vanadium phosphate, lithium mixed metal phosphates, metal sulfides, and combinations thereof.
[0087] The cathode active material particles are coated with a thin layer of carbon before mixing the cathode active material particles with the graphene suspension. For instance, micron, sub-micron, or nano-scaled NMC-811 particles may be mixed into a solution containing a carbon precursor e.g., sugar, in water
24
orphenolic resin in a solvent. The liquid component is removed from the resulting mixture suspension or paste to obtain sugar- or resin-coated NMC particles. These coated particles are heat-treated at a temperature of 500 – 1000 ? to obtain carbon-coated particles. [0088] As illustrated in Figure 2 is a schematic illustration of a working mechanism of the graphene battery, where the graphene sheets are uniformly disposed on the exterior surface of the cathode particulates, naturally forming a 3-D network of electron-conducting paths when these particulates are packed together to form the final electrode. The rate/number of charges transferred in this case is higher than what is observed in the case of a standard lithium-ion system. The carbon-coated particles are added to the graphene solution or suspension. The one or more graphene sheets embrace and protect the primary particles to form the particulates that are easier to handle in a real cathode production environment. The notion that the exterior surface is embraced with highly conductive graphene sheets implies that these sheets can naturally form a 3D network of electron-conducting paths when multiple particulates are packed together in a cathode. The graphene sheets embrace and protect the primary particles to form the particulates that are more uniform in particle sizes and are larger in average size (~10 µm) than the primary particles. Size of primary particles is conducive to electrode production using existing production equipment and found to lead to cathodes that have a higher tap density (weight per volume of the electrode), which is a very important parameter for a cathode.
[0089] In accordance with an embodiment of the present subject matter relates to a process for production of a precursor solution of pristine graphene directly from graphite particles. The process comprising the steps of: preparing a precursor solution or suspension containing pristine nano-graphene platelets (NGPs) dispersed in a liquid medium using direct ultrasonication. Further, the process may involve removing some of the liquid from the precursor solution or suspension; and adding a desired amount of a cathode active material or a precursor to the cathode active material to obtain a precursor mixture suspension or precursor solution. Furthermore, the process may involve removing the liquid
25
from the suspension to obtain a precursor mixture solid; and thermally and chemically converting the precursor mixture solid to the graphene-enhanced cathode particulate. [0090] Accordingly, for the preparation of cathode, multiple graphene enhanced particulates are mixed with a binder solution e.g., PVDF in NMP, to obtain a slurry or paste. A desired amount of the slurry or paste is coated onto a current collector, allowing the liquid to evaporate and leaving behind an electrode bonded to a surface of a current electrode. For examples, lithium nickel manganese cobalt oxide particles embraced by graphene sheets are added to a solution containing a solvent (NMP). The resulting paste is coated onto an aluminium foil as the current collector to form a coating layer of 50 – 500 µm thick. By allowing the solvent to vaporize one obtains a positive electrode (cathode) for lithium battery.
[0091] In the aforementioned examples, the starting material for the preparation of NGPs is a graphitic material that is 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. The graphite oxide is prepared by dispersing or immersing a laminar graphite material e.g., powder of natural flake graphite or synthetic graphite, in an oxidizing agent.
[0092] In another embodiment, the pristine graphene material is preferably produced by one of the following three processes comprising the steps of: intercalating the graphitic material with a non-oxidizing agent followed by a thermal or chemical exfoliation treatment in a non-oxidizing environment; subjecting the graphitic material to a supercritical fluid environment for inter-graphene layer penetration and exfoliation; dispersing the graphitic material in a powderform to an aqueous solution containing a surfactant or dispersing agent to obtain a suspension and subjecting the suspension to direct ultrasonication. Particularly preferred step comprising of 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
26
alkali or alkaline metal; and a chemical exfoliation treatment e.g., by immersing potassium-intercalated graphite in ethanol solution. [0093] Typically, when the nanographene sheets formed into a thin film with a thickness no greater than 100 nm, the resulted nanographene sheets exhibit an electrical conductivity of at least 10 S/cm, often higher than 100 S/cm, and, in many cases, higher than 1000 S/cm. The resulting NGP powder material has a specific surface area of from approximately 300 m/g to 2600 m/g and, in many cases, comprises single-layer graphene or few-layer graphene sheets. When thes graphene sheets are combined with cathode active material particles to form graphene-enhanced particulates, the particulates exhibit an electrical conductivity no less than 10 S/cm. The graphene component is in an amount of from 0.01% to 30% by weight based on the total weight of graphene and the cathode active material combined.
[0094] The below Table 1 shows the superior performance of the presently invented graphene-enhanced particulates wherein the primary cathode active particles are pre-coated with carbon. These data show that NGPs, using the graphene enhanced particulate approach, impart dramatically higher conductivity (by 60-fold) to the carbon-coated electrodes. The NGP-enhanced electrodes exhibit conductivity values that are 3-5 times higher than those of the electrodes containing 2% CNTs or carbon black (Super-P). These results are very surprising and could not have been predicted based on existing knowledge. No prior artwork has shown electrode performance that is anywhere near what the present invention has achieved.
Table 1: Approximate dimensions, resistance, and resistivity of several dry electrodes containing carbon-coated cathode particles and various conductive additives (no more than 2%).
27
C-coated
Cathode
particles
Cathode particles coated with carbon and super-P
Cathode particles coated with carbon and CNT
Graphene incorporated cathode particles (pre-coated with carbon)
Thickness (cm)
0.5
0.45
0.46
0.465
Area (cm2)
5.2
5.5
4.3
6.2
Resistance (in plane) O
375
25
20
4
Resistance (through plane) O
18.7
2
3
0.12
Resistivity (in plane, ?) O cm
198
12
9.2
1.9
Resistivity (through plane, ?) O cm
200
14
19.4
1.98
[0095] The following examples clearly illustrate the overall process of the fabrication of the nanostructured graphene and the cathode composite material for the proposed Li-ion battery cell, and the subsequent steps to compile the secondary cell, for energy storage.
[0096] EXAMPLE 1: Graphite oxide is prepared by oxidation of graphite flakes with sulfuric acid, Sodium nitrate, and potassium permanganate at a ratio of 4:1:0.05 at 30 ? for 48 hours, according to the method of Hummers U.S. Pat. No. 2,798, 878, Jul. 9, 1957. Upon completion of the reaction, the mixture is poured into deionized water and filtered. The sample is washed with 5% HCl solution to
28
remove most of the sulfate ions and residual salt and repeatedly rinsed with deionized water until the pH of the filtrate was approximately 7. The intent is to remove all sulfuric and nitric acid residue out of graphite interstices. The slurry was dried and stored in a vacuum oven at 60 0C for 24 hours. The dried, intercalated (oxidized) compound is exfoliated by placing the sample in a quartz tube that is inserted into a horizontal tube furnace pre-set at 1050 0C to obtain highly exfoliated graphite. The exfoliated graphite is dispersed in water along with a 1% surfactant at 45 0C in a flat-bottomed flask and the resulting graphene oxide (GO) suspension is subjected to ultrasonication for a period of 15 minutes. The cathode active materials studied in this example include lithium cobalt oxide, lithium iron phosphate, and lithium mixed metal phosphate in a fine particle form. For the preparation of graphene-enhanced particulates, an amount of a selected cathode active material powder is added to a desired amount of GO suspension to form a precursor mixture suspension with a solid content of approximately 10% by weight. After thorough mixing in an ultrasonication reactor, the suspension is spray-dried to form the graphene-enhanced particulates. [0097] EXAMPLE 2: Several dry electrodes containing graphene-enhanced particulates, such as lithium cobalt oxide or lithium iron phosphate primary particles embraced by graphene sheets, are prepared by mixing the particulates with a liquid to form a paste without using a binder, such as PVDF. The paste is cast onto a surface of a piece of glass, with the liquid medium removed to obtain a dry electrode. Another dry electrode is prepared by directly mixing the metal oxide/phosphate primary particles with graphene sheets in an identical liquid to form a paste without using a binder. Again, the paste is cast to form a dry electrode. The dry electrodes are evaluation of the effect of various conductive additives on the electrical conductivity of an electrode. For comparison purposes, several additional dried electrodes are prepared under exactly identical conditions, and the paste in each case is made to contain the same cathode active particles, but a comparable amount of other conductive additives: multi-walled carbon nano-tubes (CNTs), carbon black (Super-P), a CNT/Super-P mixture at an 1/1 ratio, and a GO/Super-P mixture at an 1/1 ratio. Corresponding “wet' electrodes for
29
incorporation in a battery cell are made to contain a PVDF binder. These electrodes are made into full cells containing graphite particles or lithium metal as an anode active material. [0098] It may be further noted that the cathode active material that can be used in the presently invented electrode is not limited to lithium cobalt oxide and lithium iron phosphate. There is no particular limitation on the type of electrode active materials that can be used.
G) ADVANTAGES OF INVENTION
[0099] The present invention allows the cathode active materials relatively safe in the lithium-ion batteries for electric vehicle applications. The present invention allows the conductive additive or modifier which helps multiple cathode active materials aggregate into the nanographene-enhanced particulates. Hence, the nanographene-enhanced particulates are more conducive to the formation of a 3D network of electron-conducting paths, imparting exceptional conductivity to the cathode and enabling the cathode to become high rate capable.
[00100] It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the claims presented in the complete specification or non-provisional application.
30 ,CLAIMS:I /We claim:
1. A nano graphene-enhanced particulate for use as a lithium battery cathode active material:
wherein said particulate is formed of a single or a plurality of graphene sheets and a plurality of fine cathode active material particles with a size smaller than 10 um, and the graphene sheets and the particles are mutually bonded or agglomerated into said particulate with a graphene sheet embracing said cathode active material particles; and
wherein said particulate has an electrical conductivity no less than 10-2 S/cm and said graphene is in an amount of from 0.01% to 30% by weight based on the total weight of graphene and the cathode active material combined.
2. The particulate as claimed in claim 1, wherein the graphene amount is from 0.1% to 20% by weight of the total weight of graphene and the cathode active material combined.
3. The particulate as claimed in claim 1, wherein the graphene amount is from 0.5% to 10% by weight of the total weight of graphene and the cathode active material combined.
4. The particulate as claimed in claim 1, wherein said particulate has an electrical conductivity greater than 10-2 S/cm.
5. The particulate as claimed in claim 1, wherein said particulate is spherical in shape.
6. The particulate as claimed in claim 1, wherein said graphene comprises single-layer graphene or few-layer graphene, wherein said few-layer graphene is defined as a graphene platelet formed of less than 10 graphene planes.
7. The particulate as claimed in claim 1, wherein said cathode active material is selected from the group consisting of lithium nickel manganese cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium vanadium oxide, lithium-mixed metal oxide, lithium iron phosphate, lithium manganese phosphate, lithium
31
vanadium phosphate, lithium mixed metal phosphates, metal sulfides, and combinations thereof. 8. The particulate of claim 1, wherein said cathode active material particles in a particulate have a dimension smaller than 1 µm.
9. The particulate of claim 1, wherein said cathode active material particles in a particulate have a dimension smaller than 100 nm.
10. The particulate of claim 1, wherein said cathode active material is selected from the group consisting of lithium iron phosphate, lithium manganese phosphate, lithium nickel manganese cobalt oxide, lithium mixed metal phosphates, and combinations thereof and wherein said cathode active material contains sub-micron or nano-scaled particles with a size less than 1 µm.
11. The particulate of claim 1, wherein said cathode active material is selected from the group consisting of lithium iron phosphate, lithium manganese phosphate, lithium nickel manganese cobalt oxide, lithium mixed metal phosphates, and combinations thereof and wherein said cathode active material contains sub-micron or nano-scaled particles with a size less than 100 nm.
12. The particulate as claimed in claim 1 comprising a carbon material in electronic contact with said cathode active material and a graphene sheet.
13. The particulate as claimed in claim 1 comprising a carbon material coated on at least one of said cathode active material particles, wherein said carbon material is selected from polymeric carbon, amorphous carbon, chemical vapor deposition carbon, carbon black, acetylene black, activated carbon, fine expanded graphite particle with a dimension smaller than 100 nm, artificial graphite particle, natural graphite particle, or a combination thereof.
14. The particulate as claimed in claim 1 said particulate is prepared from a process comprising: (a) preparing a precursor mixture of graphene or graphene precursor with a cathode active material or active material precursor, and (b) thermally and/or chemically converting said precursor mixture to said graphene-enhanced cathode particulate.
15. The particulate as claimed in claim 1 said step of preparing a precursor mixture comprises preparing a suspension of graphene or graphene precursor in a
32
liquid medium and mixing a cathode active material or active material precursor in said suspension to form a multi-component suspension. 16. The particulate as claimed in claim 1, wherein the process further comprises a step of drying said multi-component suspension to form said precursor mixture.
17. The particulate as claimed in claim 1, wherein the process further comprises a step of drying said multi-component suspension to form said precursor mixture using a spray-drying, spray pyrolysis, or fluidized-bed drying procedure.
18. The particulate as claimed in claim 1, wherein said step of converting comprises a sintering, heat-treatment, spray-pyrolysis, or fluidized bed drying or heating procedure.
19. The particulate as claimed in claim 1, wherein said step of converting comprises a procedure of chemically or thermally reducing said graphene precursor to reduce or eliminate oxygen content and other non-carbon elements of said graphene precursor.
20. The particulate as claimed in claim 1, wherein said graphene precursor contains graphene oxide or graphene fluoride.
21. The particulate as claimed in claim 1, wherein said graphene in said particulate has an oxygen content less than 25% by weight.
22. The particulate as claimed in claim 1, wherein said graphene in said particulates has an oxygen content less than 5% by weight.
23. The particulate as claimed in claim 1, wherein said graphene in said particulates has an oxygen content in the range of 5% to 25% by weight.
24. The particulate as claimed in claim 1, wherein a method of preparing said precursor mixture comprises:
A) dispersing or exposing a laminar graphite material in a fluid of an intercalant and/or an oxidant to obtain a graphite intercalation compound (GIC) or graphite oxide (GO);
B) exposing the resulting GIC or GO to a thermal shock at temperature for a period of time sufficient to obtain exfoliated graphite or graphite worms;
33
C) dispersing the exfoliated graphite or graphite worms in a liquid medium containing an acid, an oxidizing agent, and/or an organic solvent at a desired temperature for a duration of time until the exfoliated graphite is converted into a graphene oxide dissolved in the liquid medium to form a graphene solution; and
D) adding a desired amount of said cathode precursor material to said graphene solution to form said precursor mixture in a suspension, slurry or paste form.
25. The particulate as claimed in claim 1, wherein a step of preparing said precursor mixture comprises:
(a) preparing a suspension containing pristine nanographene platelets (NGPs) dispersed in a liquid medium;
(b) adding an acid and/or an oxidizing agent into said suspension at a temperature for a period of time sufficient to obtain a graphene solution or suspension; and
(c) adding a desired amount of cathode active material or precursor in the graphene solution or suspension to form a paste or slurry.
26. The particulate as claimed in claim 1, wherein a lithium battery cathode comprising multiple nano graphene-enhanced cathode particulates.
27. The particulate as claimed in claim 13, wherein a lithium battery cathode comprising multiple nano graphene-enhanced cathode particulates.
28. The particulate as claimed in claim 1, wherein a lithium battery cathode comprising multiple nano graphene-enhanced cathode particulates, wherein said multiple particulates are packed together with graphene sheets forming a three-dimensional electron-conducting pathway.
29. The particulate as claimed in claim 1, wherein a lithium battery cathode comprising multiple nano graphene-enhanced cathode particulates, wherein said multiple particulates are packed together with graphene sheets and said carbon material together forming a three-dimensional electron-conducting pathway.
30. The particulate as claimed in claim 26, wherein a lithium battery comprising an anode, a cathode, a separator disposed between the anode and the cathode, and electrolyte in physical contact with both the anode and the cathode.
34
31. The particulate as claimed in claim 27, wherein a lithium battery comprising an anode, a cathode, a separator disposed between the anode and the cathode, and electrolyte in physical contact with both the anode and the cathode.
32. The particulate as claimed in claim 28, wherein a lithium battery comprising an anode, a cathode, a separator disposed between the anode and the cathode, and electrolyte in physical contact with both the anode and the cathode.