The novel technology relates generally to materials science, and, more particularly, to graphene-enhanced oxyfluoride cathode materials.
Iron oxyfluoride (FeOF) is a reaction-reversable electrode material, but suffers from two major issues, low rate performance and structural instability. The electrochemical performance (specific capacity/energy, rate performance, cycle life, etc.) of FeOF has been characterized at very low current density (on the order of 50 mA/g, or 0.1 C), which is far too low for most practical applications, as power sources for EV and portable electronics typically provide 1.0 C and ⅓ C, respectively. The observed low rate performance and low specific capacity/energy is due to the low electric conductivity of FeOF, which is typical of most metal oxides and metal fluorides. Additionally, the slow Li+ ion diffusion within the FeOF nanoparticles also contributes to the low rate performance.
The other drawback mentioned above is structural instability. Although FeOF has been shown to exhibit the reversible conversion for (de)lithiation, FeOF typically undergoes about 50 cycles at 0.1 C (50 mA/g) with much lower initial capacity, on the order of 440 mAh/g. FeOF exhibits a rapid drop in capacity from initial capacity, such as from 650 mAh/g to 400 mAh/g after only a few cycles. FeOF typically loses about 90% capacity over 100 or so cycles, even at an extremely small current density (such as on the order of 0.005 mA/cm2). Although the conversion and reconversion reaction of FeOF is reversible, such huge capacity loss at such extremely small current density (which is close to the equilibrium state) is indicative of structural instability of FeOF as the cause of the performance degradation.
Thus, there is a need for stabilized FeOF electrode material having increased specific capacity and/or electrical conductivity as well as increased cycle life with decreased degradation over time. The present novel technology addresses these needs.
Graphene sheets are incorporated into the nanostructure of metal oxyflourides to render the conversion reaction of metal oxyfluorides (e.g., FeOF) reversible as well as increase specific capacity, specific energy, rate capability, cycleability, and/or safety. Relatively low electric conductivity, crystal structure stability and the relocation of metal nanoparticles are common issues for all of metal oxides and metal oxyfluorides, and the incorporation of graphene sheets into nanostructure of these oxides and oxyfluorides allows for tailoring the structure of materials and developing next generation of battery materials for energy storage and other applications. By incorporating graphene sheets into the FeOF microstructure/nanostructure, the theoretical specific capacity (590 (2 e−) and 885 (3 e−) mAh/g), 1720 Wh/kg, and 150 cycles (with 80% initial capacity) have been observed.
One advantage of the graphene modification of FeOF materials is that a simple effective incorporation of the graphene sheets can significantly change the materials in terms of morphology, structure and performance. The incorporation of graphene, in particular functionalized graphene, provides an effective and robust tool for tailoring the materials to achieve specifically desired properties (i.e. surface hydrophobic, intra/interparticle electric conductivity, particle size and morphology, and the like) while producing a material that remains cost effective.
High-quality graphene with high surface area may be made by the simple oxidation of natural graphite powders.
According to an embodiment of the present disclosure, a composite electrode material is provided including a plurality of graphene sheets, and a plurality of FeOF nanoparticles anchored to each graphene sheet.
According to another embodiment of the present disclosure, a battery is disclosed including the composite electrode material.
According to yet another embodiment of the present disclosure, a method of manufacturing a composite electrode material is disclosed including comprising: preparing a solution comprising FeSiF6 and graphene oxide in a solvent; heating the solution to convert the FeSiF6 to FeOF; and reducing the graphene oxide to graphene.
The above-mentioned and other features and advantages of this disclosure, and the manner of attaining them, will become more apparent and will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein:
For the purposes of promoting an understanding of the principles of the novel technology, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates.
Lithium ion batteries (LIBs) play a critical role in our life today. Ranging from portable electronics (i.e. cellphone, iPad, laptop, etc.), medical devices (e.g. pacemakers, Holter monitor, remote patient monitoring systems, sensors etc.), the transportation (e.g. electric vehicles (EVs) and hybrid electric vehicles (HEVs)), military equipment (i.e. unmanned underwater vehicles, radio, etc.) and many other applications, all needs the power supplies with high specific capacity/energy. Lithium has the lowest density among all metals, 0.534 g·cm−3, is the lightest metal, and has the most negative reduction potential, −3.05V (vs. standard hydrogen electrode potential). The low density and the negative potential give lithium metal the highest theoretical specific capacity, 3861 mAh g−1 (compared to 372 mAh g−1 of the carbon anode in LIBs) while the negative potential allows the construction of a battery with high open-circuit voltage. This combination of high capacity and negative potential consequently leads to high energy density batteries. However, lithium metal suffers the poor cycle life when Li metal is used as the anode in a rechargeable battery coupled with a metal oxide as the cathode. This poor cycle life is caused by the Li dendrites which grow with the charge/discharge cycle and eventually, penetrate through the separator to reach cathode and consequently, causing the short-circuit, thermal-run away and smoke and/or fire.
In 1991, the first commercial LIB was introduced, which replaced the Li metal with graphite anode and used the LiCoO2 as the cathode. When the cell is charging, Li+ ions leave the LiCoO2 electrode (i.e., delithiation of LiCoO2), diffuse through the liquid electrolyte and enter the graphite (i.e., lithiation of graphite). When the cell is discharging, the Li+ ions diffuse out the graphite, diffuse through the liquid electrolyte, then enter the CoO2. In such a process, the Li+ ions always remain in ionic state, while the graphite and CoO2 experience the oxidation state change. The LiCoO2 gradually becomes CoO2 and at the end of the charging process, LiCoO2 completely transforms into CoO2 while the Co3+ ions in LiCoO2 gradually changes to Co2+ ions in the CoO2 host and at the end of the charge process, only Co2+ ions exist in the CoO2 host. Such a battery behaves like a rocking chair in which Li+ ions swing back and forth between graphite anode and CoO2 cathode during the charge and discharge process. (Practically, only ½ Li can be reversibly intercalated/deintercalated). Therefore, LIB is also called “Rocking Chair Battery”.
The specific capacity and specific energy of a LIB cell depend on the anode and cathode materials. With the rapid development of the portable electronics and the EVs/HEVs, the demand for higher specific energy batteries becomes more urgent than ever. In order to meet these demands, it is highly desired to develop novel electrode materials. As anode materials offer a higher Li-ion storage capacity (e.g. theoretical specific capacity, 372 and 4200 mAh/g for graphite, and nanostructured Si, respectively) than cathodes do (e.g. theoretical specific capacity, 272 and 175 mAh/g for LiCoO2 and LiFePO4, respectively), the cathode material is the limiting factor in the performance of LIBs.
Most of the cathode materials for LIBs are transition metal compounds, oxides, or complex oxides. Such transition metal compounds have layered (e.g. LiCoO2), spinel (e.g. LiMn2O4) or olivine (e.g. LiFePO4) crystal structures, and transition metal cations typically display four- and/or six-fold coordination with oxygen anions, anionic clusters, or ligands. Lithium ions are inserted via an electrochemical intercalation reaction. While lithium ions occupy the space between adjacent layers or unoccupied octahedral or tetrahedral sites, an equal number of electrons enter the available d orbitals of the transition metal cations in the host crystal. Essentially, the oxidation state of metal ions keep change with the (de)insertion accompanying the phase change of these compounds while the Li+ ions remain in ionic state. These materials have some common characteristics: (1) chemical stability, (2) structural stability and (3) channels allowing the effective diffusion of Li ions within the solid oxides. The chemical stability of the cathode material ensures that the host of the cathode does not decompose during the (de)lithiation process while structural stability allows the repeated (de)intercalation of Li+ ions into the lattices of the host materials. Channels within the materials lead to the high rate (de)lithiation process within the materials, which in turn is essential for the high rate performance of LIBs. To achieve the high specific energy (Wh/kg), cathode materials need to have high specific capacity (mAh/g), which is the capacity for storing Li+ ions within the metal oxides. Additionally, the cathode materials are desired to have high potential (vs. Li/Li+) because the specific energy is the product of cell voltage and specific capacity.
The factors for high specific energy cathode materials are (1) high specific capacity (capacity of Li+ ion storage), and (2) the high electrochemical potentials (vs. Li/Li+). Two approaches have been taken for developing high specific energy cathode materials: (1) materials with transition metal ions capable of multi valence changes (e.g. V and Mn) and (2) materials with high potentials (vs. Li+/Li). For instance, V2O5 has the theoretical specific capacity of 443 mAh/g and is the highest in all cathode materials for Li+ intercalation reaction. This is because V5+ in the V2O5 molecule can have up to 3 oxidation state changes, V5→V4+, V4→V3+ and V3→V2+; correspondingly, V2O5 has the high ion storage capacity, namely, each V2O5 molecule can hold up to 3 Li+ ions. The V2O5 materials have not been used as practical LIB cathode materials due to (1) the low electric conductivity, and (2) structural stability, which are common for most of metal oxides. The low electric conductivity leads to the (1) low specific capacity because some of regions with slippery grain boundaries of V2O5 in a particle can't be reached at normal charge/discharge rate (i.e. 0.3 or 1.0 C rate), a low utilization leads to a typical specific capacity, around 250 mAh/g; on other hand, (2) the V2O5 cathode can't be operated at high charge/discharge rate. In addition, (3) some irreversible phase changes accompany the charge/discharge processes, which leads to poor cycle life. Overall, for developing high specific capacity cathode materials, multi valence metal-based compounds are critical.
Another approach for achieving high specific energy is to develop the metal oxides with high voltage. Many metal oxides have been investigated, such as Li1-xMn2-yMyO4, Li1-xCo1-yMyO2, Li1-xNi1-y-zCOyMzO4 (M=Mg, Al . . . ). Recent work focuses on the ternary metal oxides, Li1-xNi1-y-zCoyMzO4 (LiNCM, M=Mn, Mg, Al . . . ) which have very high voltages. However, there are some structural stability issues as they undergo deep discharge and cause the rapid performance decay upon cycling. In addition, the NCM based cathodes typically require much higher charging voltage to reach the fully charged state. Such high charging voltage requires the use of the electrolyte systems with up to 6 V electrochemical windows which needs solvents with much wider electrochemical window (e.g. fluorinated carbonates, sulfone based solvents and nitrile based solvents) or additives. There is a potential safety hazard when a LIB cell of NCM is charged at such high voltage, which could lead to the decomposition of the organic solvent in the electrolyte and consequent thermal-run away.
Transition-metal oxides, fluorides and oxyfluorides have attracted a lot of interest due to their ability to deliver high electrochemical specific energy arising from 2-3 electrons transferred.
There are quite few choices of 3d-transition metals for multi valence metal oxides, namely Ti, V, Cr, Mn, Fe, Co, Ni Cu, etc. With the exception of their electrochemical potentials and Li ion storage capacity (specific capacity), the toxicity and cost are two other important factors. Among all of these transition metals, Fe is the most abundant, nontoxic, and low-cost materials. However, Fe in either Fe2O3 or FeF3, can only have one oxidation state change (i.e. Fe3+→Fe2+) during the intercalation reaction. To further increase its specific capacity, one would logically think that, if the oxidation state can be further changed from 1 valence change (i.e. Fe3+→Fe2+) to 3 valence change, namely, Fe3+→Fe2+, Fe2+→Fe, this in turn, will lead to total 3 Li+ ion storage capacity. This 3-valence change results in the reduction of Fe3+ to Fe0, which is called the conversion reaction as shown below.
FeF3+3 Li↔Fe+3LiF (theoretical capacity: 712 mAh/g, E0=3.44 V)
Among the transition-metal oxides, Fe2O3 has attracted much attention due to its high theoretical specific capacity (1005 mAh/g), low cost, and non-toxicity. However, Fe2O3 has relatively low potential vs. Li/Li+, and the Fe2O3 particles suffer from rapid capacity fading because of the low conductivity and strong aggregation during the charge and discharge processes. On the other hand, FeF3 has much higher potential 0.75 V higher than Fe2O3), but lower capacity (712 mAh/g).
In order to combine the advantages of both materials, a mixed-anion FeOF was proposed as a promising candidate because it has a high theoretical specific capacity of 885 mAh/g (3-electron process) and 590 mAh/g (2-electron process), leading to an exceptionally high theoretical specific energy of 2938 Wh/kg and 1958 Wh/kg for 3- and 2-electron reactions respectively. However, the electrochemical performance of FeOF is drastically different in practice due to its low electronic conductivity and poor structure stability during charge/discharge cycling process.
The performance characteristics of various cathode materials are summarized in Table 1 below.
The first cycle of FeOF lithiation and delithiation is different from the following cycles. During the lithiation, FeOF undergoes the intercalation of Li+ ions into FeOF first, followed by the conversion into a lithiated nanocrystalline rock salt (Li—Fe−O−F) structure, metallic Fe and LiF phases. During the delithiation, the rock salt phase does not disappear, but co-exists up to the end of delithiation with an amorphous rutile type phase formed initially by the reaction of LiF and Fe. In addition, a de-intercalation stage is still observed at the end of reconversion similar to a single-phase process despite the coexistence of these two (nanocrystalline rock salt and amorphous rutile) phases. After the first cycle, the process is the intercalation followed by the conversion into a nanoscale intermixing of the two (amorphous rutile and nanocrystalline rock salt) phases, finally a nanocomposite of metallic Fe0, LiF, and rock salt Li—Fe−O(—F).
The structural/chemical ordering of FeO0.7F1.3 is illustrated in
For the fully delithiated electrodes, the FeOF has the structure of the nanoscale intermixing of amorphous rutile and nanocrystalline rock salt phases and such a structure is stable up to 20 cycles. However, upon further cycling, the amount of amorphous rutile phase decreased while the amount of rock salt phase increased gradually, suggesting the incomplete reconversion reactions with cycle number. Additionally, the solid electrolyte interphase (SEI) layer grows with the cycles, which is mainly composed of LiF. Fe2+ and Fe nanoparticles were trapped in the SEI layer with cycles. Finally, upon cycling, the combined progressive increase in Fe2+ content and insulating LiF (from SEI and conversion product) is responsible to capacity loss. The catalytic interaction of nanosized metallic particles (i.e., Fe0) with the electrolyte, which is believed to be the main reason underlying the decomposition of the electrolyte on the particle's surface, contributes to the capacity loss.
As noted above, FeOF presents two major issues, (1) low rate performance and (2) structural stability. The electrochemical performance (specific capacity/energy, rate performance, cycle life, etc.) of FeOF is poor at very low current density (i.e. 50 mA/g, or 0.1 C), which makes FeOF a poor choice for practical applications, as power sources for EV and portable electronics usually require for batteries working at 1.0 C and ⅓ C, respectively. The cause of the low rate performance and low specific capacity/energy is due to the low electric conductivity of FeOF, which is common for most metal oxides and metal fluorides. Additionally, the slow Li+ ion diffusion within the FeOF nanoparticles also contributes to the low rate performance. Another issue is the structural stability. FeOF is characterized by reversible conversion for FeOF (de)lithiation, FeOF is typically only good for 50 or so cycles at 0.1 C (50 mA/g) with much lower initial capacity, 440 mAh/g. FeOF also experiences a rapid capacity drop from initial capacity, 650 mAh/g to 400 mAh/g after only a few cycles. Although the conversion and reconversion reaction of the formed FeOF is reversible, such huge capacity losses at such extremely small current densities (which are close to the equilibrium state) suggests the FeOF structural stability is the cause of the performance degradation.
The performance of an electrode material is always rooted in its structure. Understanding the structure change of FeOF and the mechanism of (de)lithiation allows developing FeOF cathode materials.
To overcome the above-described challenges of FeOF, conducting graphene matrices have been introduced into the FeOF nanoparticles. The graphene may improve the electric conductivity of the FeOF particles, provide a substrate for the FeOF particles, and absorb the volume changes and to improve the structural stability of the electrodes.
The low electric conductivity of FeOF is one of the major causes for the low rate and low specific capacity. In addition, to facilitate the fast Li+ ion conversion reaction and increase the utilization of FeOF materials during conversion reaction, the high surface area of FeOF particles is desired for Li+ ion access, namely, uniform and small nanoparticles. To increase the reversibility of the conversion reaction, it is helpful to provide a substrate for the FeOF particles to anchor on so that the formed Fe nanoparticles at the end of the lithiation process do not delocalize, allowing that the intermixing of the amorphous rutile and nanocrystalline rock salt phases and the metallic Fe nanoparticles (core-shell structure with O-rich rock salt shell and bcc-Fe0 core) can go back to the core-shell structure of O-rich rock salt shell and F-rich rutile as shown in
Graphene has been considered as one of the most attractive carbon materials for its excellent charge carrier mobility, mechanical robustness and thermal and chemical stability. As shown in
Graphene can be prepared using the chemical or thermal reduction of graphene oxide (GO), which is a layered stack of oxidized graphene sheets with different functional groups. Thus, GO can be easily dispersed in the form of single sheet in water at low concentrations. The cost of GO is very low (e.g. estimated $10-20/kg from chemical oxidation of nature graphite method), hence the incorporation of graphene into the metal oxide nanoparticles should not result in significant additional cost since only very small amount of graphene is used. The key is to control the low concentration of GO to avoid the restacking of the GO sheets, which leads to the diminishing of the unique properties of graphene.
An exemplary solution-based solvothermal method is shown in
FeSiF6·6H2O→FeF2+SiF4 (gas)+6H2O(gas) (1)
FeF2+O2 (gas)→FeOF (2)
The FeOF product was then freeze-dried/spray-dried and heat-treated in a tube furnace with temperature of about 200-350° C. for about 1-12 hours to reduce the GO to graphene. The various method steps, including the temperatures, times, concentration of precursor FeSiF6, and concentration of graphene oxide, may be controlled and optimized to obtain FeOF nanoparticles with small diameter.
In the illustrated embodiment of
As shown in
As shown in the SEM and TEM images of
As shown in the XAS spectra of
As shown in the TEM diffraction patterns of
As shown in the EELS images of
As discussed above with respect to
In certain embodiments, the functional groups may be covalently grafted onto the surface of the graphene sheets through a diazonium salt via a diazonium reaction. The diazonium reaction-based functionalization is a simple and cost-effective way to transform the pure graphene sheets into hierarchical and functional materials that can provide the desired properties (i.e. hydrophobicity, Li+/e− conductivity, nanoparticle dispersion and local electric field, etc.) and the functionalized graphene sheets for FeOF nanoparticles to anchor. In addition, such a method is easy for large-scale manufacturing.
The cycle life data for different functional groups is shown in Table 2 below. The —COOH functional group had a positive impact on cycle life, whereas the —OH functional group had a negative impact on cycle life, possible due to the stereo effect of the charged groups.
Except for the loss of Fe nanoparticles in the fully lithiated FeOF due to the dissolution, the further cycling of FeOF causes the formation of excess LiF, which is insulated and prevents further delithiation, which is another cause of capacity fading. In certain embodiments, an ultra-thin polymer coating or protection layer with good electronic conductivity may be uniformly coated over the surface of a FeOF nanoparticle. An exemplary coating layer is PANI, which is electrically conductive (6.28×10−9 S/m) and its conductivity can be enhanced by HBr doping, 4.60×10−5 S/m (4% HBr doping). Other suitable polymeric coatings include PBI, PEO, PPO, and/or mixtures thereof, for example. The graphene sheets may hold the coated FeOF nanoparticles together to protect the FeOF nanoparticles from Fe dissolution and LiF formation, and, consequently, extend the cycle life. The coating may also be transformed into a carbon layer through the pyrolysis to enhance the electric conductivity.
One interesting aspect of the present novel technology arises from the synergetic approach of (1) incorporating graphene sheets into FeOF nanostructure to make the FeOF reversible conversion materials with excellent performance, and (2) interaction with the (de)lithiation mechanism of metal oxyfluorides using synchrotron XAS and TEM to guide the material development. Unlike most of LIB materials such as LiCoO2, LiFePO4, LiMn2O4 and V2O5 etc., which are either toxic (i.e. V2O5), expensive (i.e. LiCoO2) and/or of low specific energy (i.e. LiFePO4) and/or of low cycle life (LiMn2O4), the proposed novel graphene incorporated nanostructured FeOF/graphene composites are non-toxic, low cost, and high specific energy (i.e. 1720 Wh/kg, 3× of LiCoO2, LiMn2O4 and LiFePO4), which are the most promising cathode materials for the next generation of LIBs. The incorporation of graphene sheets into metal oxides and metal oxyfluorides was shown to improve the electric conductivity, manipulate the particle morphology, and maintain their structural integrity during the (de)lithiation process, which opens a new avenue for effectively developing novel conversion and other electrode materials.
Advantages of the approach of graphene modified materials over the current electrode materials include (1) a simple effective incorporation of the graphene sheets can significantly change the materials in terms of morphology, structure and performance; (2) the incorporation of graphenes, particular the functionalized graphenes provides an effective and robust tool for tailoring the materials to achieve the desired properties (i.e. surface hydrophobic, intra/interparticle electric conductivity, particle size and morphology, etc.) and (3) the incorporation is cost effective: high-quality graphene of high surface area may be produced made by the simple oxidation of natural graphite powders. Graphene incorporated FeOF composites may greatly advance the battery industries, consequently, leading the break-through on portable electronics, and electrification of the automobiles as required by many countries.
This unique and non-conventional approach of incorporating graphene sheets into metal oxides and metal oxyfluorides to tailor the materials' morphology and structure makes the FeOF/graphene composite reversible conversion materials with a high specific capacity/energy, high rate, high cyclability, and high safety. This approach can be realized by simply incorporating graphene oxide sheets in the FeOF synthesis process, leading to a novel, graphene modified and nanostructured FeOF composites, yielding a reversible conversion material with a specific energy 3× that of the current LiFePO4 with at least 1000 cycles that is ready for commercial applications.
While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.
FeOF was recently found to be a conversion type cathode material for LIBs because of its high theoretical capacity (885 mAh). Rutile structure FeOF was both environmental friendly and economic. During the charge and discharge process, the valence of iron changes from 3+ to 0, which means that it can deliver 3 electrons. However, the cyclability of this cathode was still too poor. In order to increase the cycle life of FeOF, it is very important to clearly elucidate the failure mechanism by clearly understanding the atom environment in real-time during the cycling.
Synchrotron X-ray near-edge structure (XANES) is very helpful for illustration of the local structure and state of charge of the element of interest. With the help of XANES, in-situ characterization of FeOF cathode is made to better elucidate real-time local structure and valence change at different state of charge (SOC) and depth of discharge (DOD) in order to better illustrate the mechanism of the iron ion evolution and FeOF failure mechanism. FeOF was prepared and mixed with carbon black, PVDF and NMP to form a uniform slurry. The cathode was prepared by coating the slurry on aluminum foil. The in-situ test coin cell was assembled by FeOF cathode, Celgard separator and lithium foil anode and was sealed in our home-made coin cell shell. K-edge of Fe was measured during the in-situ characterization to observe the valence change and local structure evolution during the discharge and charge process between the voltage range of 4V and 1V.
The in-situ XANES spectrum is plotted in
In conclusion, by applying in-situ XANES technique, we can clearly visualize the valence and local structure evolution mechanism during real working condition and get our preliminary conclusion that the charge/discharge process of FeOF battery contains two typical processes: Li+ intercalation occurs at high voltage range and conversion process occurs at lower voltage range.
Synthesis of the Generally Sphere-Like Graphite Nanoparticles:
An isotropic petroleum pitch was heat-treated in a furnace. This furnace included a cylindrical stainless steel reactor, fitted with an anchor-type stirrer and a thermocouple connected to a temperature controller/microprocessor. The reactor was heated using a cylindrical furnace. The reactor was loaded with 400 grams of the precursor pitch and heated at a rate of 3 degrees Celsius per minute until the desired soak temperature of 420 degrees Celsius was achieved. The precursor pitch particles were generally spherical in shape and were soaked at 420 degrees Celsius for 2 hours. During the heat treatment, an agitation of 70 rpm was maintained as was a flow of nitrogen gas at a rate of 0.5 cubic meters per hour for removal of any evolved volatile materials.
In order to separate the spherical particles from the parent pitch, first the heat treated pitch was mixed with wash oil and then filtered at 100 degrees Celsius, followed by three successive washes with toluene, at 75 degree Celsius in a water bath, and then centrifuged for separation. The separated particles were dried and successively oxidized in air at 200 degrees Celsius for 5 hrs, carbonized at 1000 degrees Celsius for 15 min, and graphitized at 2800 degrees Celsius.
Preparation of the Nano Graphite Particles:
The acid bath was composed of nitric acid (70%), sulfuric acid (98%) and perchloric acid (60%) present in a ratio of 1:6:1 (v/v), respectively. For each batch of graphite nanoparticles, 5 g graphite powder was placed into the etching acid bath and heated to about 200 degrees Celsius while being constantly stirred. Two samples were prepared by heating for 1 hour and 2 hours, respectively. For the separation of the etched samples, the mixture was centrifuged at 15000 rpm and each sample was washed with distilled water for 5 times.
Characterization:
High-resolution TEM images were obtained using a transmission electron microscope. The electron beam accelerating voltage of the TEM was 200 kV for all images. All the samples were suspended in ETOH, drop-cast onto a lacey-carbon TEM grid (SPI), and the solvent was allowed to completely evaporate. The morphologies of the graphite spheres and the etched samples were examined using cold field emission scanning electron microscopy. The crystalline structure of the graphite spheres and the etched samples were investigated using X-ray wide angle diffraction. The diffractometer utilized Cu Kα radiation (40 kV and 30 mA). The data were collected as continuous scans, with a step size of 0.020 (20) and a scanning rate of 20 (20)/min between 10-900 (20). The surface chemistry of the raw graphite spheres and acid etched samples was analyzed using X-ray photoelectron spectrometry. The spectrometer had an Al Kα X-ray source. An electron flood gun for charge neutralization and a hemispherical analyzer with 8 multichannel photomultiplier detectors was employed for analysis. The area of analysis was 700×300 microns in size. The XRD results for confirmed the material to be essentially pure graphite from the 100 and 101 characteristic peaks at 42.22 and 44.39 degrees, respectively, and the TEM diffraction pattern results indicated a layer d-spacing of about 3.4 Å, as compared to the ideal d-spacing for graphite of 3.35 Å, confirming graphite.
A GO solution was prepared using a modified Hummer's method. 2 grams of graphite flakes were mixed with 10 mL of concentrated H2SO4, 2 grams of (NH4)2S2O8, and 2 grams of P2O5. The obtained mixture was heated at 80° C. for 4 hours under constant stirring. Then the mixture was filtered and washed thoroughly with DI water. After drying in an oven at 80° C. overnight, this pre-oxidized graphite was then subjected to oxidation using the Hummer's method. 2 grams of pre-oxidized graphite, 1 gram of sodium nitrate and 46 mL of sulfuric acid were mixed and stirred for 15 minutes in an iced bath. Then, 6 grams of potassium permanganate was slowly added to the obtained suspension solution for another 15 minutes. After that, 92 mL DI water was slowly added to the suspension, while the temperature was kept constant at about 98° C. for 15 minutes. After the suspension has been diluted by 280 mL DI water, 10 mL of 30% H2O2 was added to reduce the unreacted permanganate. Finally, the resulted suspension was centrifuged several times to remove the unreacted acids and salts. The purified GO were dispersed in DI water to form a 0.2 mg/mL solution by sonication for 1 hour. Then the GO dispersion was subjected to another centrifugation in order remove the un-exfoliated GO. The resulted GO dilute solution could remain in a very stable suspension without any precipitation for a few months.
Two FeSiF6-6H2O solutions were heated to 120° C. and then to 200° C. under 02 gas flow. To one sample, a dilute GO solution was added and further processed to form FeOF particles with 10 wt. % graphene. The resulting blank FeOF and FeOF/graphene materials were assembled as cathodes in coin cells using Li metal anodes and dielectric separators with electrolytes including 1.0 M LiPF6 in a 3:7 by weight solvent mixture of EC and EMC for electrochemical testing.
This application claims the benefit of U.S. Provisional Patent Application No. 62/636,304, filed Feb. 28, 2018, titled “OXYFLUORIDE CATHODES AND A METHOD OF PRODUCING THE SAME,” the entire disclosure of which is hereby incorporated herein by reference in its entirety.
Number | Date | Country | |
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62636304 | Feb 2018 | US |