Hard carbon composite for alkali metal-ion batteries

Abstract

A method is provided for fabricating a graphene-doped, carbohydrate-derived hard carbon (G-HC) composite material for alkali metal-ion batteries. The method provides graphene oxide (GO) dispersed in an aqueous solution. A carbohydrate is dissolved into the aqueous solution and subsequently the water is removed to create a precipitate. In one aspect, the carbohydrate is sucrose. The precipitate is dehydrated and exposed to a thermal treatment of less than 1200 degrees C. to carbonize the carbohydrate. The result is the formation of a graphene-doped, carbohydrate-derived hard carbon (G-HC) composite. Typically, the G-HC composite is made up of graphene in the range of 0.1 and 20% by weight (wt %), and HC in the range of 80 to 99.9 wt %. The G-HC composite has a specific surface area of less than 10 square meters per gram (m.sup.2/g). A G-HC composite suitable for use in alkali metal-ion batteries electrodes is also provided.

Claims

1. A method for fabricating a graphene-doped, carbohydrate-derived hard carbon composite material for alkali metal-ion batteries, the method comprising: providing graphene oxide (GO) dispersed in an aqueous solution; dissolving a carbohydrate into the aqueous solution; removing water to create a precipitate; dehydrating the precipitate; exposing the precipitate to a thermal treatment; and, forming a graphene-doped carbohydrate-derived hard carbon (G-HC) composite.

2. The method of claim 1 wherein forming the G-HC composite includes forming a G-HC composite comprising graphene in a range of 0.1 and 20% by weight (wt %) and HC in a range of 80 to 99.9 wt %.

3. The method of claim 1 further comprising: in response to the thermal treatment, carbonizing the carbohydrate.

4. The method of claim 1 wherein adding the carbohydrate to the aqueous solution includes adding sucrose to the aqueous solution.

5. The method of claim 1 wherein exposing the precipitate to the thermal treatment includes exposing the precipitate to a temperature of less than 1200 degrees Centigrade (C).

6. The method of claim 5 wherein exposing the precipitate to the thermal treatment includes performing the thermal treatment in an inert gas environment using a gas selected from a group consisting of nitrogen and argon.

7. The method of claim 1 wherein forming the G-HC composite includes the G-HC composite having a specific surface area (SSA) of less than 10 square meters per gram (m.sup.2/g).

8. The method of claim 1 wherein dispersing the GO into the aqueous solution includes performing a process selected from a group consisting of sonication and stirring.

9. The method of claim 1 wherein dehydrating the precipitate includes heating at a temperature of less than 250 degrees C.

10. A graphene-doped, carbohydrate-derived hard carbon composite battery electrode, the battery electrode comprising: a current collector; carbonaceous materials overlying the current collector comprising: a graphene-doped, carbohydrate-derived hard carbon (G-HC) composite; and, a binder material.

11. The battery electrode of claim 10 wherein the ratio of G-HC composite to the carbonaceous materials is in a range of 70 to 100% by weight (wt %).

12. The battery electrode of claim 10 wherein the carbonaceous materials further comprise a conductive carbon additive.

13. The battery electrode of claim 12 wherein the ratio of conductive carbon additive in the carbonaceous materials is in a range of 0 to 30 wt %.

14. The battery electrode of claim 10 wherein the ratio of binder material to the carbonaceous materials is in a range of 3 to 20 wt %.

15. The battery electrode of claim 10 wherein the binder material includes a polymer.

16. The battery electrode of claim 10 wherein the G-HC composite is a graphene-doped, sucrose-derived hard carbon composite.

17. The battery electrode of claim 10 wherein the G-HC composite has a specific surface area (SSA) of less than 10 square meters per gram (m.sup.2/g).

18. A composite material for use as an alkali metal-ion battery electrode, the composite material comprising: graphene-doped, carbohydrate-derived hard carbon composite.

19. The composite material of claim 18 wherein the graphene-doped, carbohydrate-derived hard carbon composite is a graphene-doped sucrose-derived hard carbon composite.

20. The composite material of claim 18 wherein the G-HC composite has a specific surface area (SSA) of less than 10 square meters per gram (m.sup.2/g).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a plot depicting powder X-Ray Diffraction (XRD) patterns of hard carbon (HC) and G-HC composite.

(2) FIGS. 2A and 2B are potential-capacity profiles for G-HC composite and HC electrodes, respectively, cycled at a current density of 20 milliamps per gram (mA/g) in a potential range of 2.0 V to 0.01 V.

(3) FIGS. 3A through 3D depict potential-capacity profiles for HC (FIGS. 3A and 3B) and G-HC composite (FIGS. 3C and 3D) electrodes cycled at a current density of 20 mA/g in a potential range of 2.0 V to 0.01 V.

(4) FIG. 4 is a plot depicting the nitrogen adsorption and desorption isotherms of HC and G-HC composite.

(5) FIG. 5 is a graph depicting the charge capacities of G-HC composite and HC at various current densities (20 mA/g.fwdarw.40 mA/g.fwdarw.100 mA/g.fwdarw.200 mA/g.fwdarw.500 mA/g.fwdarw.1000 mA/g.fwdarw.500 mA/g.fwdarw.200 mA/g.fwdarw.100 mA/g.fwdarw.20 mA/g) in a potential range of 2.0 V to 0.01 V.

(6) FIG. 6 is a plot depicting potential-capacity profiles for the G-HC composite electrode containing conductive carbon additive (10 wt %) cycled at a current density of 20 mA/g in a potential range of 2.0 V to 0.01 V.

(7) FIG. 7 is a plot depicting cycling and rate performance (charge/discharge capacities) for the G-HC composite electrode containing conductive carbon additive (10 wt %) cycles at various current densities (20 mA/g.fwdarw.40 mA/g.fwdarw.100 mA/g.fwdarw.200 mA/g.fwdarw.500 mA/g.fwdarw.1000 mA/g.fwdarw.500 mA/g.fwdarw.200 mA/g.fwdarw.100 mA/g.fwdarw.40 mA/g.fwdarw.20 mA/g) in a potential range of 2.0 V to 0.01 V.

(8) FIGS. 8A and 8B are partial cross-sectional views of a graphene-doped, carbohydrate-derived hard carbon composite battery electrode.

(9) FIG. 9 is a flowchart illustrating a method for fabricating a graphene-doped, carbohydrate-derived composite material for alkali metal-ion batteries.

DETAILED DESCRIPTION

(10) For the synthesis of graphene-doped, carbohydrate-derived (G-HC) composite materials, graphene oxide (GO) was synthesized from natural graphite flakes using a modified “Hummer's” method. Next, sucrose (2 grams) was dissolved into an aqueous dispersion of GO (25 milligrams (mg) GO dispersed in 75 milliliters (mL) H.sub.2O) with sonication for 2 hours at room temperature. The sucrose/GO solution was subsequently dried at 80° C. for 24 hours to afford a brown precipitate. The obtained precipitate was further dehydrated in air at 180° C. for 24 hours, followed by a carbonization procedure (thermal treatment) performed in a tube furnace at 1100° C. for 6 hours under an argon (Ar) atmosphere.

(11) FIG. 1 is a plot depicting powder X-Ray Diffraction (XRD) patterns of hard carbon (HC) and G-HC composite. In a control experiment, sucrose was directly dehydrated and carbonized under the identical conditions used for preparing G-HC composite. After the final carbonization step, powder XRD analysis showed that G-HC composite and HC exhibited similar HC structure characterized by two broad peaks at ˜24° and 43°, which can be indexed to the (002) and (101) planes, respectively, as shown in the figure. In addition, field emission scanning electron microscope (FESEM) images showed that the G-HC composite and HC powders consisted of irregular particles with sizes ranging from 20 to 100 microns.

(12) FIGS. 2A and 2B are potential-capacity profiles for G-HC composite and HC electrodes, respectively, cycled at a current density of 20 milliamps per gram (mA/g) in a potential range of 2.0 V to 0.01 V. For the investigation of sodium-ion (Na.sup.+) storage properties, a test platform consisting of Na pellets (counter electrode), a glass fiber membrane (separator), and 1.0 mole per liter (mol/L or 1 M) sodium hexafluorophosphate (NaPF.sub.6) in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 by volume) as an electrolyte were assembled into coin cells in an argon-filled glovebox. A coin cell consists of a small metal canister that forms the body and positive terminal of a battery along with an insulated top cap that serves as the negative terminal. The working electrode consisted of G-HC composite or HC (93 percent by weight (wt %)) and polyvinylidene fluoride (PVDF, 7 wt %) as a binder. A typical mass loading of G-HC composite or HC on the current collector (aluminum (Al) foil) was ˜2.5 milligrams per square centimeter (mg/cm.sup.2). Initially, the cells were cycled at a low current density (20 mA/g) within a potential range of 2.0 V to 0.01 V. As shown in the figures, two distinct potential regions were observed in the potential-capacity profiles for the G-HC composite and HC electrodes. The slope in the first discharge curve from 1.0 to ˜0.1 V corresponds to Na.sup.+ insertion into turbostatic graphene inter-layers, as well as to the irreversible formation of solid electrolyte interface (SEI) layer, whereas the long plateau near 0 V is ascribed to Na.sup.+ insertion into the nanopores of HC. The charge profiles correspond to the removal of Na.sup.+ from HC. The overall discharge/charge capacities of G-HC composite and HC are 269/220 milliamp hours per gram (mAh/g) and 263/197 mAh/g, respectively. Taking into account the fact that the specific capacity difference between the first discharge and charge capacities corresponds to the irreversible capacity, it is evident that the irreversible capacity for G-HC composite is smaller than that of HC (18.5% vs. 24.9%).

(13) FIGS. 3A through 3D depict potential-capacity profiles for HC (FIGS. 3A and 3B) and G-HC composite (FIGS. 3C and 3D) electrodes cycled at a current density of 20 mA/g in a potential range of 2.0 V to 0.01 V. The experimental result described by FIGS. 2A and 2B was confirmed through additional cells evaluated in parallel, from which it can be concluded that G-HC composite demonstrates an average irreversible capacity of ˜17.2% (FIGS. 2A, 3C, and 3D) compared to ˜26.9% for HC (FIGS. 2B, 3A, and 3B). Furthermore, it is evident that the discharge capacity contribution (indicated by Part i in FIGS. 2A and 2B) from the slope region in HC is larger than that of G-HC composite, which is consistent with a larger irreversible capacity for HC arising from SEI formation.

(14) FIG. 4 is a plot depicting the nitrogen adsorption and desorption isotherms of HC and G-HC composite. According to previous technical reports, the enhanced reversible capacity of G-HC composite should be attributed, at least in part, to a smaller specific surface area (SSA). Therefore, N.sub.2 adsorption/desorption measurements were carried out for the purposes of determining the porosity of G-HC composite and HC materials. As shown in the figure, G-HC composite and HC show distinct isotherms. In particular, whereas HC exhibits a porous structure, G-HC composite shows the properties of a dense material. The corresponding Brunauer-Emmett-Teller (BET) SSAs of G-HC composite and HC were determined as 5.4 m.sup.2/g and 137.2 m.sup.2/g, respectively. Upon careful inspection during synthesis, it was observed that pure sucrose transformed into a “foamed” caramel (visually) following the dehydration process, whereas the sucrose/GO composite maintained its dense morphology under identical dehydration processing. A plausible reaction mechanism involves the formation of a large number of pores after removal of water in the sucrose, which arise as a consequence of bubbles produced by water vapor. In contrast, water can be effectively removed along GO sheets in the sucrose/GO composite, thereby preserving the dense structure. Following further thermal treatment (carbonization), HC prepared from dehydrated sucrose exhibits an abundance of pores. In contrast, the dehydrated sucrose/GO composite was transformed into graphene-doped HC featuring a low SSA.

(15) FIG. 5 is a graph depicting the charge capacities of G-HC composite and HC at various current densities (20 mA/g.fwdarw.40 mA/g.fwdarw.100 mA/g.fwdarw.200 mA/g.fwdarw.500 mA/g.fwdarw.1000 mA/g.fwdarw.500 mA/g.fwdarw.200 mA/g.fwdarw.100 mA/g.fwdarw.20 mA/g) in a potential range of 2.0 V to 0.01 V. Initially, both the HC and G-HC composite cells were cycled at a current density of 20 mA/g for 10 cycles. Under these conditions, HC and G-HC composite cells demonstrate average capacities corresponding to 198 mAh/g and 221 mAh/g, respectively. Upon increasing the current density to 40 mA/g, G-HC composite maintains a higher capacity (176 mAh/g) compared to HC (140 mAh/g). However, HC and G-HC composite demonstrate similar capacities after further increasing the current density (100 to 1000 mA/g). This observation is reasonable since rapid cycling benefits from a larger SSA to facilitate the rapid transport of Na.sup.+ (insertion/desertion) between carbon matrix and electrolyte. Therefore, G-HC composite shows a lower rate performance compared to other (highly porous) carbons. Finally, the respective charge capacities can be restored to 225 mAh/g (G-HC composite) and 205 mAh/g (HC) upon reducing the current density back to 20 mA/g, which is indicative of stable cycling performance for HCs derived from sucrose.

(16) FIG. 6 is a plot depicting potential-capacity profiles for the G-HC composite electrode containing conductive carbon additive (10 wt %) cycled at a current density of 20 mA/g in a potential range of 2.0 V to 0.01 V.

(17) FIG. 7 is a plot depicting cycling and rate performance (charge/discharge capacities) for the G-HC composite electrode containing conductive carbon additive (10 wt %) cycled at various current densities (20 mA/g.fwdarw.40 mA/g.fwdarw.100 mA/g.fwdarw.200 mA/g.fwdarw.500 mA/g.fwdarw.1000 mA/g.fwdarw.500 mA/g.fwdarw.200 mA/g.fwdarw.100 mA/g.fwdarw.40 mA/g.fwdarw.20 mA/g) in a potential range of 2.0 V to 0.01 V. In order to investigate the impact of conductive carbon additive on the behavior and rate performance of G-HC composite, carbon black (TIMCAL Super C45, 10 wt %) was included in the composition of the G-HC composite electrodes (G-HC composite:carbon black:PVDF=8:1:1). As evident in FIG. 6 for G-HC composite electrodes containing carbon black (10 wt %), the first discharge and charge capacities increase to 373 mAh/g and 295 mAh/g, respectively. Moreover, the rate performance is greatly enhanced and is characterized by excellent cycling stability (FIG. 7). Overall, the higher capacity and better rate performance (versus no conductive carbon additive) can be attributed to better conductivity owing to the inclusion of conductive carbon. At the same time, the irreversible capacity increases slightly to 22.5% (from 17.2% without conductive carbon), which is due to the higher SSA of the carbon black (45 m.sup.2/g).

(18) In summary, a facile approach for the synthesis of a HC composite material with low SSA (5.4 m.sup.2/g) has been demonstrated through the introduction of GO into the sucrose solution prior to thermal treatment. Prototype cells employing this low SSA HC electrode achieve a low irreversible capacity (17.2%) and exhibit stable cycling performance. Upon addition of conductive carbon (10 wt %) into the electrode composition, both the capacity and rate performance can be significantly improved. Overall, the technology described herein provides a promising strategy for realizing cost-effective and large-scale production of HC featuring low SSA and low corresponding irreversible capacity. Moreover, the scope of the core synthesis protocol can be extended for the purposes of preparing additional graphene-doped carbon materials with low SSAs.

(19) Graphene-HC composites can be employed as high performance anode materials in sodium-ion battery applications (and others). Technology advantages include (1) low irreversible capacity due to low SSA and (2) robust rate and cycle life performance.

(20) FIGS. 8A and 8B are partial cross-sectional views of a graphene-doped, carbohydrate-derived hard carbon composite battery electrode. The battery electrode 800 comprises a current collector 802, which is typically a highly conductive metal such as aluminum (Al) or copper (Cu). Carbonaceous materials 804 overlie the current collector 802 comprising a graphene-doped, carbohydrate-derived hard carbon (G-HC) composite, schematically represented as reference designator 806 and a binder material schematically represented as 808. Carbonaceous materials are materials rich in carbon. In one aspect, the G-HC composite 806 is a graphene-doped, sucrose-derived hard carbon. The G-HC composite 806 has a SSA of less than 10 square meters per gram (m.sup.2/g).

(21) Typically, the ratio of G-HC composite 806 to the total amount of carbonaceous materials 804 is in the range of 70 to 100% by weight (wt %). The ratio of binder material 808 to the carbonaceous materials 804 is in the range of 3 to 20 wt %. Some typical binder materials include polymers such as polyvinylidene fluoride (PVDF), polytetraflurorethylene (PTFE), sodium carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), etc. In general, the role of the binder, which is usually polymeric in nature, is to sequester the carbonaceous materials together and provide both form factor and mechanical integrity to the corresponding electrode.

(22) In one aspect, as shown in FIG. 8B, the carbonaceous materials 804 further comprise a conductive carbon additive 810. Typically, conductive carbon additives are higher SSA materials employed for the purpose of improving the overall electrical conductivity of the electrode. The ratio of conductive carbon additive 810 in the carbonaceous materials 804 is in the range of 0 to 30 wt %.

(23) FIG. 9 is a flowchart illustrating a method for fabricating a graphene-doped, carbohydrate-derived composite material for alkali metal-ion batteries. Although the method is depicted as a sequence of numbered steps for clarity, the numbering does not necessarily dictate the order of the steps. It should be understood that some of these steps may be skipped, performed in parallel, or performed without the requirement of maintaining a strict order of sequence. Generally however, the method follows the numeric order of the depicted steps. The method starts at Step 900.

(24) Step 902 provides graphene oxide (GO) dispersed in an aqueous solution. Step 904 dissolves a carbohydrate into the aqueous solution. A “carbohydrate” may be defined as a macromolecule that includes carbon, hydrogen, and oxygen atoms in its molecular structure. As used herein, a “carbohydrate” may be synonymous with “saccharides” including monosaccharides, disaccharides, oligosaccharides, and polysaccharides. In one aspect, the carbohydrate is sucrose. The process of creating a dispersion of GO in Step 902 may be aided with the use of sonication or stirring. Similarly, subsequent dissolution of the carbohydrate (e.g. sucrose) in Step 904 can be accomplished using stirring. Step 906 removes water to create a precipitate. For example, the water may be removed from the solution with the dispersed GO and dissolved carbohydrate by heating in an air atmosphere at a temperature of 100 degrees C. or less. Accordingly, residual water may still be present in the precipitate formed in Step 906. Step 908 dehydrates the precipitate. For example, in Step 908 the precipitate may be heated at a temperature of less than 250 degrees C., in an air environment.

(25) Step 910 exposes the precipitate to a thermal treatment, typically at a temperature of less than 1200 degrees C. In one aspect, the thermal treatment is performed in an inert gas environment using a gas such as nitrogen or argon. In response to the thermal treatment, Step 912 carbonizes the carbohydrate. As used herein, “carbonization” is a thermal process through which an organic material or compound is converted into carbon or a carbon-containing residue, which may proceed via pyrolysis. As used herein, pyrolysis refers to a thermal treatment performed in the absence of oxygen. Due to an absence of an oxidant (oxygen), thermally-treated materials (including carbohydrates) decompose into combustible gases and carbonaceous materials. By definition, organic materials and compounds contain carbon atoms in their molecular structures and include carbohydrates. Step 914 forms a graphene-doped, carbohydrate-derived hard carbon (G-HC) composite. As used herein, a composite or composite material is composed of two or more components with different physical/chemical properties that, as a result of being combined, furnish a new material characterized by properties that are different than any of the original components. Typically, the G-HC composite comprises graphene in the range of 0.1 and 20 wt % and HC in the range of 80 to 99.9 wt %. In one aspect, the G-HC has a SSA of less than 10 m.sup.2/g.

(26) A graphene-doped, carbohydrate-derived hard carbon (G-HC) composite and associated fabrication process have been provided. Examples of particular materials and process details have been presented to illustrate the invention. However, the invention is not limited to merely these examples. Other variations and embodiments of the invention will occur to those skilled in the art.