Tailored recovery of carbons from waste tires for enhanced performance as anodes in lithium-ion batteries

Abstract

Morphologically tailored pyrolysis-recovered carbon black is utilized in lithium-ion battery anodes with improved capacity as a potential solution for adding value to waste tire-rubber-derived materials. Micronized tire rubber was digested in a hot oleum bath to yield a sulfonated rubber slurry that was then filtered, washed, and compressed into a solid cake. Carbon was recovered from the modified rubber cake by pyrolysis in a nitrogen atmosphere. The chemical pretreatment of rubber produced a carbon monolith with higher yield than that from the control (a fluffy tire-rubber-derived carbon black). The carbon monolith showed a very small volume fraction of pores of widths 3–5 nm, prominent nanoporosity (pore width < 2 nm), reduced specific surface area, and an ordered assembly of graphitic domains. Electrochemical studies revealed that the recovered-carbon-based anode had a higher reversible capacity than that of graphite. Anodes made with a sulfonated tire-rubber-derived carbon and a control tire-rubber-derived carbon exhibited an initial coulombic efficiency of 71% and 45%, respectively. The reversible capacity of the cell with the sulfonated tire rubber-derived carbon as the anode was 390 mA h g⁻¹ after 100 cycles, with nearly 100% coulombic efficiency. Our success in producing a higher performance carbon material from waste tire rubber for potential use in energy storage applications adds a new avenue to tire rubber recycling.

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1. Introduction

Proper disposal and recycling of worn-out rubber tires prevent the threats large piles of them pose to the environment and to public health and safety. In the past such tires were mostly placed in landfills, but in the past few decades, uses for ground rubber tires have been found. By 2009, of the nearly 290 million scrap tires generated in the United States that year, almost 90% of them were consumed either as fuel; as additives in civil engineering applications; as additives to plastics, rubbers, and asphalt; or in agriculture through various usages.¹ Tires contain significant quantities of organic materials and have a significantly higher heat value than coal.² Shredded tires can be incinerated, but the typically incomplete combustion discharges smoke, toxic chemicals, and obnoxious odors. Therefore, reclamation of the materials for other value-added uses is desired.

Reclaiming the rubbery materials from waste tires requires separating the steel and textile reinforcing carcass, shredding the rubber chunks, and chemically or mechanically breaking down the cross-linked networks in the vulcanized rubber chunks, often termed devulcanization.³ Although use of reclaimed rubber offers processing advantages, loading a significant quantity of reclaimed rubber in a new rubber formulation is impossible without a deleterious effect on product quality.^4,5 Despite the fact that devulcanization produces the highest-quality reclaimed rubber, it has other drawbacks, including reduced process safety and the foul smell of the disulfide-type chemicals required in the process.⁶ As an alternative, waste tire rubbers are usually cryogenically pulverized into small micron-sized rubber particles.⁷ Shredded rubber pieces are also ground in ambient conditions to produce powder buffings. Powdered tire rubbers are useful as filler in various low-cost rubber or plastic products.^8–11 Use as low-cost additives often requires chemical modification and surface compatibilization of the tire rubber particles with the matrix, which is not always a cost-effective solution.^11,12 Some effective recommendations include bitumen modification^13,14 and production of activated carbon.^15,16 Tire-derived hydrocarbon fuel is another use of waste tires.

With the current demand to restart dry shale gas production in the United States, although sometimes controversial,¹⁷ the future market potential for tire-derived fuel needs additional support. For example, utilizing the pyrolyzed residue from waste tires after fuel extraction would boost the economics of the recycling process. In fact, the pyrolytically recovered carbon black from waste tires is one of the major products obtained from tire recycling. However, isolated carbon black from waste tires does not necessarily produce a good reinforcing filler for tire or other polymeric products.¹⁸ Usually high-structure carbon black made from clusters of ∼10–100 nm spherical particles is used in tire rubber formulations to enhance the mechanical properties of the product.¹⁹ Carbonaceous deposits on black particles during tire pyrolysis cause a lowering of the surface activity that can be avoided if pyrolysis is conducted at reduced pressures.²⁰ Therefore, in general, pyrolytically recovered black from waste tires does not retain the structural characteristics necessary for the reinforcing effect. Although carbon residues, as by-products of tire-derived-fuel manufacturing, do not normally find efficient use, it is important to mention here that tire-derived fuel is one of the solutions for tire recycling recommended by the US Environmental Protection Agency.²¹ Manufacturing value-added carbon residue from a portion of the recycled tire rubber could be beneficial.

Use of tire rubber materials for significantly value-added applications would be very attractive not only from the materials recovery standpoint but also from the perspective of controlling environmental hazards caused by waste tire stockpiles. Scrap tire piles are flammable, and such fires are difficult to extinguish. They also provide breeding ground for mosquitoes and other insects, and rats.²² Therefore, the search for alternative uses of waste-tire-derived materials continues, and a method that significantly enhances the performance of products containing waste-derived material would be commercially attractive.

Recovery of functionalized carbon black from used tires has been attempted with thermal plasma pyrolysis, which yields both syngas compositions and a carbon black residue with less aliphatic carbon content.²³ Carbon black materials are used commercially as a low-cost additive for various electrode applications. They can also be used in the manufacture of graphite electrodes in industrial metallurgy and as an active material in lithium-ion battery anodes. A report on electrode applications of pyrolytically recovered carbon from waste tires is available,²⁴ but it does not demonstrate any uses as being specifically for Li-ion battery anodes. It has been shown that the performance of carbon black materials in Li-ion battery anodes is dependent on the carbon morphology.^25,26

In this article we report recovery of a morphologically tailored carbon black from micronized tire rubber. Our goal was to modify the characteristics of the recovered carbon black for enhanced performance as Li-ion battery materials. Tire rubber powder was treated in a hot oleum bath to yield sulfonated rubber powder, which was then filtered, washed, and compressed into a solid cake, followed by pyrolysis in an inert atmosphere (nitrogen). The physical characteristics of the unmodified tire-rubber-derived carbon (the control carbon) were compared with those derived from the sulfonated tire rubbers (the tailored carbon). Their electrochemical performances as an anode for the Li-ion battery were tested in a half-cell configuration.

2. Results and discussion

Pyrolysis-based recovery of carbon black for its value-added use as an active material in Li-ion batteries was investigated. The results from two types of carbon materials produced by different methods from powdered tires were compared. The following two methods were used: (1) simple pyrolysis of powdered rubber at 1000 °C, under a flowing nitrogen gas atmosphere, that yields 33% carbon (control tire-rubber-derived carbon) and (2) digestion of rubber powders in a hot oleum bath (20% SO₃ at 70 °C for 12 h) to yield sulfonated rubber powder that was then filtered, washed, and compressed into a solid cake followed by pyrolysis in an inert atmosphere (sulfonated tire-rubber-derived carbon). The second method produced a carbon monolith with ∼7% higher yield than the control rubber powder when the sulfonated rubber was preheated at 160 °C. The unmodified tire rubber produced a fluffy (low-bulk-density) powder of carbon black. The isolated carbon materials were used in tests of their electrochemical performance as active anode material in a Li-ion battery. A schematic of the recovery of pyrolytic carbon black from recycled tires and its use as a lower-cost material in Li-ion batteries is shown in Fig. 1.

Fig. 1 Schematic of the recovery of carbon black from recycled tire rubber.

2.1 Characterization of tire rubbers and derived carbons

Thermogravimetric analysis (TGA) of the rubber powder samples was conducted to determine their compositional characteristics. The TGA data on the control and sulfonated tire rubbers are shown in Fig. 2(a). The first weight loss in sulfonated powder, around 150 °C, is the desulfonation step that causes the elimination of H₂SO₃, and it shifts the subsequent pyrolysis temperature of rubber to a slightly higher one. Tire formulation vulcanizates usually consist of a mixture of diene-rubbers such as polybutadiene, polyisoprene, or styrene-butadiene copolymer. During the thermal pyrolysis step (300–450 °C), diene-rubbers in the tire rubber decompose in two steps.²⁷ Desulfonation of rubbers or aliphatic hydrocarbons is known to form unsaturated moieties that are relatively better char-forming materials.²⁸ A detailed discussion of potential pyrolysis pathways for the elimination reaction of a sulfonated hydrocarbon is reported elsewhere.²⁹ Usually, desulfonation or elimination of sulfonic acid group (from rubber polymers) occurs with the formation of unsaturated groups in the hydrocarbon chain that undergo high temperature pyrolysis with a minimal loss of mass to yield carbon along with recovery of carbon black in the rubber matrix.

Fig. 2 (a) TGA thermograms of both control tire rubber and sulfonated tire rubber. (b) Derivative TGA thermograms of both control tire rubber and sulfonated tire rubber.

The derivative TGA data (see Fig. 2(b)) show that the rubber pyrolysis after desulfonation becomes a single-step weight loss that is responsible for the creation of char from the matrix material. Thus, the tailored carbon contains carbon black particles physically bound in a synthesized carbon matrix produced from sulfonated rubber. Nonsulfonated rubber powder matrix (control) does not yield carbon, and the isolated carbon black attains very low bulk density. Therefore, the control carbon is very fluffy compared with the solid monolithic carbon from the slurry of sulfonated tire rubber.

Pore textural characteristics of the carbon samples were measured volumetrically by N₂ adsorption–desorption at liquid N₂ temperatures (77 K). The Brunauer–Emmett–Teller (BET) surface area and pore size distributions were obtained by analyzing the data. Nitrogen adsorption–desorption plots for modified (sulfonated) and control tire-derived carbons and standard graphite at 77 K and pore size distribution plots obtained by applying non-local density functional theory (NLDFT) model on nitrogen adsorption plots are displayed in Fig. 3. For the control sample, the pore sizes range from 1 to 35 nm (10–350 Å) and pore width distribution is dominant with width larger than 7 nm (70 Å). However, the sulfonation (chemical pretreatment) prior to pyrolysis allows formation of a very small volume fraction of pores of width 3–5 nm (30–50 Å) and prominent microporosity with pore width less than 2 nm. We believe these pore widths are due to the gaps between the thin carbon matrix on the carbon black particles produced by the pyrolysis of a char-forming sulfonated rubber matrix. Sulfonated tire rubber particles undergo hydrogen bonding and form a slurry under wet conditions and then, finally, a cake of dry rubber that yields a relatively less porous carbon monolith. Further, sulfonated carbon black filled rubber, during pyrolysis, produce SO₂ and steam which yield activated monolith carbon composite with dominant microporosity. When the rubber is not sulfonated, it (matrix) does not yield any detectable char; thus leaves highly porous carbon black lump. Further, we noticed that sulfonation creates a relatively hard carbon monolith with a less pore volume. The surface area and comparative porosity data are shown in Table 1.

Fig. 3 (Top) Nitrogen adsorption–desorption plots for sulfonated and control tire-derived carbons and standard graphite at 77 K (inset: magnified plot for graphite powder); (bottom) pore size distribution plots obtained by applying non-local density functional theory (NLDFT) model on nitrogen adsorption plots (inset: magnified view of the same plot in region of lower pore width).

Table 1 BET surface area and porosity data of different carbon materials

Materials	BET surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)
Graphite powder	2	0.006
Control carbon	95	0.55
Tailored carbon	72	0.09

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Powder X-ray diffraction data (XRD) collected on bulk materials indicated that the carbons recovered from both unmodified and sulfonated tire rubber samples were mainly composed of poorly crystalline carbonaceous material (as shown in Fig. S1, see ESI†). The broad peak near 2θ ∼ 26.6° suggests lack of significant order in the bulk of the carbon materials. Raman spectra obtained on control and tailored carbons are shown in Fig. S2.† The modes near 1590 and 1360 cm⁻¹ correspond to, respectively, the G (ordered) and D (disordered) bands. With 512 nm laser excitation, we do not find significant alteration of nanoscale morphology in the two types of carbons through the peak intensity ratio of the G and D bands [with I(D)/I(G) ∼0.96 consistent for disordered carbon], except a little higher intensity in control carbon in the G band ranging from 1540–1580 cm⁻¹. Also, the control carbon exhibits a G band peak at 1583 cm⁻¹ whereas that of the sulfonated-tire derived hard carbon appears at 1596 cm⁻¹. Usually the G band intensity shifts to higher wavenumber value with increased order in graphitic clusters.³⁰ Sulfonation assisted graphitization of recovered carbon causes reduced intensities in the 1520–1580 cm⁻¹ range.

Transmission electron microscopy (TEM) images of the control recovered carbon and the tailored counterparts are reported in Fig. 4 and 5, respectively. Control carbon (Fig. 4) has the morphology of fused particles with irregular shapes; the selected area electron diffraction (SAED) pattern indicates that the amorphous phase dominates. Originally, sulfonated tire rubber produced a carbon monolith. Hydrogen bonding in sulfonated powder rubber allowed inter-particle fusion and compaction during compression molding. The resilient control rubber powder could not be molded into a uniform sheet by compression molding; it might require two-roll milling and/or a binder. Ground carbon produced from the sulfonated tire-rubber-derived monolith has the morphology of a uniaxial nanostructure (Fig. 5a and b). The electron diffraction pattern in the selected area indicates the presence of both crystalline and amorphous carbon materials (Fig. 5(c)). A representative TEM micrograph of crystalline nonporous graphite powder and the associated SAED pattern are shown in Fig. S3.† It is apparent that sulfonation, subsequent processing, and pyrolysis influence the alteration of the local meso-scale structure in some fraction of the recovered carbon.

Fig. 4 TEM images of control tire-rubber-derived carbon (a–c) and the corresponding selected area electron diffraction pattern (d).

Fig. 5 TEM images of sulfonated tire-rubber-derived carbon (a and b) and the corresponding selected area electron diffraction pattern (c).

2.2 Electrochemical studies on coin cells

Graphite has been used widely as an anode in commercial Li-ion batteries, as it can intercalate/deintercalate Li up to a composition of LiC₆ (at which a Li atom is present between every layer of the host graphite lattice) and benefits from low cost and ready availability.^31–35 Graphite offers a theoretical capacity of 372 mA h g⁻¹. Thus, even though carbon, in the form of high-quality graphite, has low atomic weight and low density (∼2.2 g cm⁻³), its reversible capacity is limited. Hence, low-cost materials with robust carbon architectures that can undergo unfatigued, reversible intercalation by Li ions and offer high capacity are the ones desired for electrode applications. The objective here is to compare the performances of three different anode materials: carbons derived from unmodified tire rubbers, sulfonated tire rubbers, and commercial graphite. Electrochemical studies were done by preparing CR2032 coin cells. The anode of a coin cell contained a mixture of 80 wt% active carbon material from waste tires, 5 wt% high-surface-area commercial conducting carbon, and 15 wt% polyvinylidene difluoride (PVDF) binder, and the current collector was copper foil. Lithium was used as the counter electrode. Galvanostatic charge/discharge cycling between 0 and 3.0 V was performed at room temperature under different rates using an Arbin potentiostat/galvanostat multichannel system.

The first and second discharge/charge cycles of a Li/carbon cell and cycled at a rate of 0.1 C (C = number of charge/discharge cycles in an hour) between 3.0 and 0.005 V are shown in Fig. 6(a)–(c). The initial reaction of the control tire-rubber-derived carbon with lithium, which occurred predominantly below 1 V, generated a discharge capacity of 890 mA h g⁻¹, whereas the subsequent first charge and second discharge yielded 400 and 510 mA h g⁻¹, respectively [Fig. 6(a)]. The large irreversible capacity loss between the first and second discharge reactions could be associated with the reduction of carbon surface groups followed by electrolyte reduction and formation of solid electrolyte interphase (SEI), if any.^36–38 The anodic performance of carbon recovered from sulfonated tire rubber shows the first discharge capacity around 545 mA h g⁻¹, and the reversible charge capacity is around 387 mA h g⁻¹, leading to an irreversible capacity of 158 mA h g⁻¹ [Fig. 6(b)]. However, both second discharge capacity and reversible charge capacity are around 390 mA h g⁻¹. Standard graphite based anode exhibits the first discharge capacity around 399 mA h g⁻¹, whereas the subsequent first charge and second discharge yielded 357 and 363 mA h g⁻¹, respectively [Fig. 6(c)]. Fig. 6 shows that the voltage profiles are different for sulfonated tire-derived carbon with a sloping region between 1.5 to 0.01 V than that of graphite, which has a much sharper charge/discharge potential. This could be due to the presence functional groups, large particle size and/or microporosity nature of the sulfonated tire-derived carbon.

Fig. 6 First and second charge/discharge curves of the carbon anodes at a rate of 0.1 charge/discharge cycles per hour (0.1 C); control tire-rubber-derived carbon (a), sulfonated tire-rubber-derived carbon (b), and standard graphite (c).

The cycling performance of the half-cell at a rate of 0.1 charge/discharge cycles per hour with anode containing the control carbon, sulfonated tire-derived tailored carbon, and standard graphite are displayed in Fig. 7(a)–(c), respectively. The control carbon exhibited an initial coulombic efficiency of 45%, much lower than that of the tailored carbon (71%) and graphite (∼90%). This initial coulombic efficiency data can be linked to the surface area data of the carbon (surface area: control carbon > tailored carbon > graphite) and it can also be cause by either the presence of surface functional groups or absorbed species.³⁶

Fig. 7 Cycling performance of the carbon anode at a rate of 0.1 charge/discharge cycles per hour (0.1 C); control tire-rubber-derived carbon (a), sulfonated tire-rubber-derived carbon (b), and standard graphite (c).

The lowest efficiency of the control carbon is likely due to its fluffy nature and the higher surface area. Its charge and discharge capacity decreased to ∼200 mA h g⁻¹, and the coulombic efficiency increased slowly to 99% after 45 cycles. The cycling performance of the half-cell with anode composed of tailored carbon exhibited excellent cycling performance with reversible capacity of ∼390 mA h g⁻¹ and 100% coulombic efficiency that was maintained to 100 cycles minimally [Fig. 7(b)]. This result is better than the experimental data with graphite (357 mA h g⁻¹) [Fig. 7(c)] and theoretical capacity of 372 mA h g⁻¹ for commercial hard carbon/graphite anodes.

The theoretical capacity of 372 mA h g⁻¹ is calculated for graphite which has a layered structure and lithium is intercalated into its layers. For graphitic (layered structured) soft carbon one lithium ion can be accommodated by 6 carbon atoms present in the anode. However, this theoretical value does not apply to all carbons; particularly to those which are not layered graphitic in nature. Hydrogen containing partially pyrolyzed carbon obtained from polymeric precursor at low temperature usually gives high capacity. However, such carbon does not reversibly form the original chemical structure of hydrogen containing carbon during charging when lithium is removed from the anode.³⁹ Those anodes usually decay the capacity after a few cycles. Our control carbon is an example of this type of carbon.

In the case of tailored carbon derived from sulfonated tire-rubber the carbon exhibits significant microporosity. Also, the material has some ordered structure due to the formation of sulfonation and desulfonation induced unsaturated hydrocarbon structure. Microporous layers of ordered carbons can accommodate lithium ions not only on intercalated layers but also at the pores that may come from folded single layer of graphite sheet like structure. Single layer carbon that creates microporosity can accommodate more lithium atom in the form of Li_xC₆ where x > 1. Further, our tailored carbon is synthesized from diene-rubber matrix containing embedded carbon black through sulfonation, a mechanism which yields crosslinked networked structure of polymer during thermal desulfonation. In such crosslinked carbon matrix very likely lithium will have more insertion position in the structure. Hence, it is normal that hard carbons will have larger capacity than graphite as anode materials.³⁹ The charge/discharge curves of three samples in Fig. 6(a)–(c) clearly show the capacity differences between first cycle and second cycles which is mainly caused by SEI formation. The discharge curve indicates the SEI formation starts from about 0.8 V for graphite. Both control and tailored carbon derived from tire rubber exhibit a slightly different discharge profile likely due to its significantly higher porosity than that of the graphite powder. It is well known that the porous carbons have a different release rate of lithium from anode during charging.^40,41 Macroporous carbon-based anode⁴⁰ has less capacity than that involving microporous carbon.⁴¹

The rate performance of the control carbon anode [Fig. 8(a)] clearly shows a capacity of ∼100 mA h g⁻¹ at 1 C and only ∼40 mA h g⁻¹ at 5 C, which is significantly less than the widely used commercial carbon anodes.³⁴ A typical anode composed of active graphite materials exhibits a reversible capacity of 357 mA h g⁻¹. Under identical conditions, the relative drop in capacity of anode materials after too many charge/discharge cycles is likely due to the changes in carbon morphology, the formation of interphase layers, and the resulting reduced conductivity. Based on results obtained on anodes composed of the control carbon, the half-cell anode performance appears to be poor.

Fig. 8 Rate performance of the control tire-rubber-derived carbon anode (a) and tailored carbon from sulfonated tire-rubber (b).

The rate performance of the tailored carbon anodes [Fig. 8(b)] was clearly excellent, with ∼270 mA h g⁻¹ at 1 C, 160 mA h g⁻¹ at 5 C, and over 50 mA h g⁻¹ at 10 C, which are much higher than that obtained from the control tire-rubber-derived carbon. This result probably occurred because (1) the tailored carbon displayed both crystalline and amorphous carbon characteristics with distinct microporosity, but the control carbon displayed predominantly amorphous characteristics; and (2) the conductivity of the control carbon is lower, owing to the contrasting morphologies discussed earlier. The superior electrochemical performance obtained from the tailored carbon indicates that this material, originating from waste tires, has potential for use as an anode material in practical Li-ion battery applications.

3. Experimental

Pulverized tire rubber powder in the size range 80–120 μm was donated by Lehigh Technologies, Inc., Georgia. Tire rubber consists of a polymer mixture of natural rubber, butadiene rubber, and styrene-butadiene rubber. The powdered recycled rubber, containing a cross-linked rubber mix (45%), carbon black (33%), inorganic filler and vulcanization activator (10%), and residual extractable and volatile materials with a specific gravity of 1.15 g cm⁻³, was used for the pyrolytic recovery of carbon black. The powder rubber samples were heated in a tubular furnace under a flowing nitrogen gas atmosphere at 1000 °C. The temperature of the furnace was ramped up from room temperature to 1000 °C at 10 °C min⁻¹; upon reaching 1000 °C, it was held constant for 15 min, and then the furnace was cooled down to room temperature and the carbon residue was collected. The sample is termed as the control carbon. The yield of carbon black was 33%. The commercial graphite with the mass-median-diameter of 20 μm was obtained from MTI Corp., Richmond, CA, USA.

In another case the tire rubber powder was treated with fuming sulfuric acid containing 20 wt% free SO₃ gas at 70 °C for 12 h. The tire rubber slurry was then filtered on a Buchner funnel with a sintered glass disc (fritted glass funnel) using an aspirator followed by washing with distilled water. The non-silica inorganic residues, such as ZnO or steel belt residues, are usually removed during oleum treatment and subsequent washing. The washed sulfonated tire rubber cake was then pressed between Teflon sheets under a hot plate inside a compression mold at 110 °C to remove moisture and to obtain a thick (2 mm) molded sheet, followed by pyrolysis in a tubular furnace under a flowing nitrogen gas atmosphere at 1000 °C. The temperature of the furnace was ramped up from room temperature to 1000 °C at 10 °C min⁻¹; upon reaching 1000 °C, the temperature was maintained for 15 min with the sample soaking inside. The furnace was allowed to cool down to room temperature and the environment was maintained under a flowing nitrogen gas before the monolithic carbon sample was removed. The sample is termed as the sulfonated tire-rubber-derived carbon or the tailored carbon. The yield of carbon based on the as-received material (non-sulfonated rubber) was 40%.

Thermogravimetric analysis (TGA) of the control and tailored tire rubber samples was performed in a TGA Q500 (TA Instruments) at a 10 °C min⁻¹ heating rate up to 1000 °C under nitrogen. The porosity characteristics of the recovered carbons were investigated by nitrogen adsorption–desorption at −196 °C (77 K) and pressure up to an ambient condition in a Qunatachrome Nova 2000 analyzer. The pore size distributions in the carbon samples were obtained by employing non-local density functional theory (NLDFT) using the data reduction software of the instrument. Transmission electron micrographs of carbon materials were obtained with an FEI Titan 60/300S S/TEM at 60 kV. The TEM specimens were prepared by directly dispersing the carbon sample to lacey carbon TEM grids. Powder X-ray diffraction data (XRD) were acquired on an Empyrian powder diffractometer equipped with Cu K_α radiation, a Ni K_β filter, and a PIXcel detector over the range 5° to 100° two theta with a 0.013° step size. Raman spectra of control carbon and tailored carbon were collected with a WITec Raman spectrometer with 512 nm laser excitation.

Electrochemical studies were performed by first preparing CR2032 coin cells. The coin half-cells were assembled in an argon-filled glove box, using recycled carbon as the working electrode and metallic lithium foil as the counter electrode. The anode was prepared by casting a slurry containing 80 wt% ground recycled carbon material, 5 wt% conducting commercial carbon (super C45) obtained from TIMCAL, and 15 wt% polyvinylidene difluoride (PVDF) binder in n-methyl-2-pyrrolidone (NMP) solvent onto a copper foil. The standard electrolyte for Li-ion batteries consisted of a solution of 1.0 M LiPF₆ in ethylene carbonate (EC)–dimethyl carbonate (DMC)–diethyl carbonate (DEC) (1 : 1 : 1 by volume). Galvanostatic charge/discharge cycling between 0 and 3.0 V was performed at room temperature under different rates using an Arbin potentiostat/galvanostat multichannel system.

4. Conclusions

A method for recovering carbon from recycled tire rubber powder includes a sulfonation step followed by pyrolysis. The tire rubber powder material was contacted with a sulfonation bath containing oleum to produce a sulfonated material. The sulfonated material after pyrolysis produced a carbon black containing composite product comprising a glassy carbon matrix phase having carbon black dispersed therein. The pretreatment of the rubber in the oleum bath produced a carbon monolith with higher yield than that occurring in control tire rubber. The control tire rubber formed a soft carbon black after pyrolysis. Although a sulfonation treatment prior to pyrolysis allows the formation of a very small volume fraction of pores with widths of 3–5 nm and prominent microporosity, it reduces the specific surface area in the recovered carbon and initiates the formation of ordered graphitic domains.

Our study demonstrating a rechargeable Li-ion battery electrode has shown a pathway for making lower-cost and higher-performance carbon materials with potential for use in energy storage applications from waste tire rubber. Electrochemical studies on the recovered carbon-based anode revealed a higher reversible capacity than that of commercial graphite. The reversible capacity of the cell with the tailored carbon as the anode was 390 mA h g⁻¹ after 100 cycles with nearly 100% coulombic efficiency. The control carbon exhibited 200 mA h g⁻¹ reversible capacity as the anode in a Li-ion battery configuration. This study discloses a value-added use of recycled materials and opens up new avenues to tire rubber recycling.

Acknowledgements

Research was sponsored by both the Laboratory Directed R&D Program and the Technology Innovation Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. The research (ZB, YL, MC, CAB, MPP) on battery fabrication and electrochemical testing was sponsored by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. Transmission electron microscopy research was supported through a user project supported by ORNL's Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Thanks are due to Dr Jagjit Nanda for help in acquiring Raman spectroscopy data. Authors acknowledge the generous donation of ground tire rubber from Lehigh Technologies, Inc., Georgia, USA. D.S. acknowledges the Faculty Development Award (2014–2015) from School of Engineering, Widener University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03888f

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