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Pnas201210072 17360.17365

Flexible graphene-based lithium ion batteries withultrafast charge and discharge ratesNa Lia,b,1, Zongping Chena,1, Wencai Rena, Feng Lia, and Hui-Ming Chenga,2 aShenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; and bDepartmentof Materials Science & Engineering, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China Edited* by Mildred S. Dresselhaus, Massachusetts Institute of Technology, Cambridge, MA 02139 and approved September 17, 2012 (received for review June13, 2012) There is growing interest in thin, lightweight, and flexible energy nanomaterials (16–18). Based on these strategies, electrodes storage devices to meet the special needs for next-generation, with a highly conductive pathway for electrons, a short ion dif- high-performance, flexible electronics. Here we report a thin, fusion length, and a fast transport channel for the ion flux can be lightweight, and flexible lithium ion battery made from graphene fabricated for fast charge and discharge. A series of electro- foam, a three-dimensional, flexible, and conductive interconnected chemical active materials (4, 15–18) and three-dimensional (3D) network, as a current collector, loaded with Li4Ti5O12 and LiFePO4, hybrid electrodes (5, 14) have been recently fabricated to as- for use as anode and cathode, respectively. No metal current col- semble rechargeable batteries with high charge and discharge lectors, conducting additives, or binders are used. The excellent rates. For example, Braun et al. (5) recently fabricated a mac- electrical conductivity and pore structure of the hybrid electrodes roporous nickel network for battery electrodes with ultrafast enable rapid electron and ion transport. For example, the Li4Ti5O12/ charge and discharge rates, and 76% of the specific capacity was graphene foam electrode shows a high rate up to 200 C, equivalent retained when discharged at 185 C. However, these batteries are to a full discharge in 18 s. Using them, we demonstrate a thin, based on a complicated electrode package and rigid electrode lightweight, and flexible full lithium ion battery with a high-rate structures with metals as current collectors, which makes the performance and energy density that can be repeatedly bent to devices less flexible, and they also have a low energy density.
a radius of 5 mm without structural failure and performance loss.
Recently, we have fabricated a unique 3D graphene macro- scopic structure: graphene foam (GF) (19). A GF consisting of flexible device full battery a 3D interconnected network of high-quality chemical vapordeposition-grown graphene can be used as the fast transport The development of next-generation flexible electronics (1), channel of charge carriers. The electrical conductivity of the such as soft, portable electronic products, roll-up displays, GF is estimated as high as ∼1,000 S/m, and the solid conductivity wearable devices, implantable biomedical devices, and conform- of the few-layer graphene itself within the GF is evaluated to be able health-monitoring electronic skin, requires power sources that ∼1.36 × 106 S/m . Moreover, the GF is extremely are flexible (2, 3). Similar to conventional energy storage devices, light (∼0.1 mg/cm2 with a thickness of ∼100 μm) and flexible. It flexible power sources with high capacity and rate performance possesses a high porosity of ∼99.7% and a very high specific that enable electronic devices to be continuously used for a long surface area and can be bent to arbitrary shapes without time and fully charged in a very short time are very important for breaking. In addition, the high quality and carbonaceous nature applications of high-performance flexible electronics (4–7). Lith- of graphene building blocks give the GF network excellent stability ium ion batteries (LIBs) have a high capacity but usually suffer in electrochemical environments. These features give the GF great from a low charge/discharge rate compared with another important potential for use in next-generation flexible electronics. Using theGF network as both a highly conductive pathway for electrons and electrochemical storage device, supercapacitors. Therefore, it is ions and a 3D interconnected current collector, we have de- highly desired to fabricate a flexible electrochemical energy storage veloped thin, lightweight, and flexible LiFePO system with a supercapacitor-like fast charge/discharge rate and battery-like high capacity. However, the fabrication of such an 4Ti5O12 (LTO)/GF electrodes that can simultaneously obtain high charge and discharge rates up to 200 C (where a 1-C rate energy storage device remains a great challenge owing to the lack represents a 1-h complete charge or discharge). Using these of reliable materials that combine superior electron and ion con- flexible bulk electrodes, we further assembled a thin, lightweight, ductivity, robust mechanical flexibility, and excellent corrosion re- and flexible full LIB (Fig. 1) that shows high capacity and high-rate sistance in electrochemical environments.
performance and is capable of repeated bending to a radius of Using nano-sized materials to prepare electrodes is one of the <5 mm without structural failure and loss of performance.
most promising routes toward flexible batteries. Metal oxidenanowires (8, 9) and carbon nanomaterials such as carbon Results and Discussion nanotubes (6, 10–12) and graphene paper (13) have been re- Synthesis and Characterization of Free-Standing Flexible LTO/GF and cently demonstrated for use in flexible LIBs. However, electron LFP/GF. LTO and LFP have been considered as promising anode transport in these electrodes is slow because of the relatively low and cathode materials for commercial applications in LIBs be- quality of nanomaterials (such as chemically derived graphene) cause of their safety, environmental friendliness, and high and/or high junction contact resistance between them. As aconsequence, only a moderate charge/discharge rate has beenobtained in these flexible batteries. It is generally believed that Author contributions: N.L., Z.C., W.R., F.L., and H.-M.C. designed research; N.L. and Z.C.
the charge/discharge rate of a LIB depends critically on the performed research; N.L., Z.C., W.R., F.L., and H.-M.C. analyzed data; and N.L., Z.C., W.R., migration rate of lithium ions and electrons through the elec- F.L., and H.-M.C. wrote the paper.
trolyte and bulk electrodes into active electrode materials.
The authors declare no conflict of interest.
Strategies to increase ion and electron transport kinetics in bat- *This Direct Submission article had a prearranged editor.
teries have mainly focused on seeking new electrode materials 1N.L. and Z.C. contributed equally to this work.
and designing conductive electrode structures with high ion and 2To whom correspondence should be addressed. E-mail: electron transport rates (4, 5, 14, 15), or reducing the path length This article contains supporting information online at over which electrons and lithium ions have to move by using 17360–17365 PNAS October 23, 2012 vol. 109 no. 43

Schematic of a flexible battery containing a cathode and an anode made from 3D interconnected GF.
performance. To build a flexible LIB with a high capacity and perpendicular to the surface of the GF, which not only provides fast charge and discharge rates, we integrated highly conductive, a large interfacial area for fast lithium insertion/extraction but porous, and flexible GF with LTO and LFP as anode and cath- also ensures a short solid-state diffusion length (Fig. 2 C and D ode, respectively (Fig. 1). It is worth noting that the 3D porous and The LTO sheet growth is nucleated at GF network was directly used as a highly conductive pathway for wrinkles on the GF because of their higher chemical reactivity electrons/lithium ions and current collectors for the LIB, without ). The direct growth of LTO on the GF enables the use of conventional metal current collectors, carbon black good contact and strong binding between LTO and GF, with no additive and binder. The LTO/GF and LFP/GF hybrid materials need to add any binder. As a result, no LTO nanosheets were were fabricated by in situ hydrothermal deposition of active peeled off even after repeated bending ( materials on GF followed by heating in an argon atmosphere The transmission electron microscope (TEM) image (Fig. 2E) (details in Methods and ). Notably, similar to GF, shows that the LTO nanosheets are several hundred nanometers these hybrid materials are very flexible (Fig. 2A), can be bent into in width and a few nanometers in thickness. The high-resolution arbitrary shapes without breaking, and completely preserve the TEM image in Fig. 2F indicates the high crystallinity of the 3D interconnected network structure of the GF (Fig. 2B).
nanosheets with a lattice fringe spacing of 4.8 Å, corresponding The LTO in the hybrid material has a sheet structure a few to the most stable and frequently observed (111) facet of spinel nanometers in thickness, and high-density LTO sheets stand LTO (), which is consistent with the X-ray Characterization of a free-standing flexible LTO/GF. (A) Photograph of a free-standing flexible LTO/GF being bent (2 × 2 cm2). (B and C) SEM images of the LTO/GF. (D) TEM image of the LTO/GF. (E) TEM image of the LTO nanosheets in LTO/GF. (F) High-resolution TEM image of a LTO nanosheet showing latticefringes with a spacing of 0.48 nm.
PNAS October 23, 2012 vol. 109 no. 43 17361

electrons, a short ion diffusion length, and a fast transport channel for a high Li+ flux, which provide the electrodes with a great po- tential for fast charge and discharge.
Electrochemical Behavior of the LTO/GF Anode. We investigated thelithium insertion/extraction properties of the LTO/GF material by galvanostatic charge–discharge measurements (Fig. 3) and Li4Ti5O12 and Li7Ti5O12
found that this LTO/GF hybrid material shows extremely high charge/discharge rates. At a charge/discharge rate of 0.1 C, the LTO/GF and LTO have similar specific charge/dischargecapacities. However, at charge/discharge rates of 1 C and 30 C, the LTO/GF shows a specific capacity of about 170 and 160 mAh/g, respectively, and even at a charge and discharge rate of 200 C (corresponding to an 18-s full discharge), it still retains fic capacity of 135 mAh/g, corresponding to ∼80% of the specific capacity at the 1-C rate. In contrast, the reference LTO, which was prepared using the same process but without the presence of GF, shows a capacity of almost zero at 200 C, al-though it also has a 2D sheet structure ().
Moreover, the rate performance of this LTO/GF material is much better than those reported in the literature for all kinds of conventional LTO electrodes integrated with metal foil current collectors, carbon black additive and binder, including nano- crystalline LTO, carbon-coated LTO, and LTO/multiwalled car- bon nanotube and LTO/graphene composites ( To further determine the stability of the electrode structure, we studied the change in morphology of the LTO/GF electrode after 100 charge/discharge cycles at 0.5 C and found that the LTO/ GF electrode is capable of long-term lithiation and delithiation at low rates without structural failure ).
More strikingly, the discharge curve of the LTO/GF anode at high rates (up to 200 C) shows a long, flat potential plateau, which ensures a constant power output, and therefore is very important for the commercial use of LIBs (20). The flat plateausegment is a characteristic of two-phase equilibrium (21), a room-temperature miscibility gap, as shown in Fig. 3A. In con- trast, although many nanostructured materials have been devel- oped to show high-rate performance (4, 5, 22–24), most of them exhibit a nonflat plateau with a capacitor-like charge–discharge curve at high rates. The profile change of voltage curves from flatto slope at high rates may be attributed to polarization related to the poor electrical conductivity of electrode materials, or may occur because electrode materials obey a pseudocapacitive (in- terfacial) storage mechanism instead of a bulk intercalation storage mechanism (25). The high-rate performance with a long, flat plateau in the potential profile of the LTO/GF electrode suggests excellent ion and electron transport kinetics of the LTO/GF hybrid structure. As shown in Fig. 3C, the specific ca- pacity of the flat plateau segment of the LTO/GF at 200 C is 86 Discharge rate and cyclic performance of the LTO/GF electrode. (A) mAh/g, whereas the value of the reference LTO is almost zero.
Two-phase equilibrium region of the LTO/GF with different charge/discharge Fig. 3D shows the cyclic stability of the LTO/GF at 30 C and 100 rates; C /n denotes the rate at which a full charge or discharge takes n hours.
C. Note that the capacity decreases less than 4% of the initial (B) Discharging voltage curves of the LTO/GF with different charge/discharge value after 500 cycles, demonstrating the excellent electro- rates. (C) Specific capacities of the LTO/GF and reference LTO at various chemical stability of this free-standing flexible electrode.
charge/discharge rates within a flat plateau segment shown in B. (D) In general, in a commercial LIB, metal current collectors Capacities of the LTO/GF charged/discharged at constant 30-C and 100-C (mainly copper, ∼10 mg/cm2 and aluminum, ∼5 mg/cm2, with rates for 500 cycles.
a thickness of ∼10–30 μm), conducting additives, and bindingagents are indispensable, and these account for 30–50% of the diffraction measurements (JCPDS 49–0207; electrode weight. Bulk electrodes constructed of either metal (5, In addition, similar to the pristine GF (19), the Raman 14) or pyrolytic carbon (26) have recently been developed forfast charge and discharge, but metal current collectors are still spectra of a free-standing LTO/GF material shows a strongly needed for facilitating electron transport from the electrode to suppressed defect-related D band (), in- the cell. These metal and carbonaceous components of bulk dicating the overall high quality of graphene in LTO/GF. This electrodes make a battery cell heavy and nonflexible. In contrast, also suggests that no defects were introduced in GF during the in our electrode, the 3D flexible and conductive interconnected LTO synthesis process, guaranteeing a very high electrical con- GF network acts not only as a highly conductive pathway for ductivity of the GF. The above features of this 3D porous LTO/ electrons/lithium ions but also as an ultralight current collector GF hybrid electrode produce a highly conductive pathway for (down to ∼0.1 mg/cm2) (without

the use of conductive additives, binding agents, and metal cur- than those of LFP/chemically derived graphene hybrid electro- rent collectors. Impedance spectroscopy was used to characterize des integrated with aluminum foil current collectors, carbon the contact and charge-transfer resistance of the free-standing black additive, and binder (27, 28). This high-rate performance is LTO/GF electrode (The values of the much better than that of a flexible LFP/multiwalled carbon contact resistance Rs and charge-transfer resistance Rct of the nanotube-yarn electrode, which only retained a capacity of ∼50 LTO/GF electrode were 6.6 and 67.1 Ω, respectively, which are mAh/g at a 10-C rate (12). Furthermore, at a 10-C charge/dis- significantly lower than those of the reference LTO coated on an charge rate, 98% capacity retention was obtained after 500 cycles aluminum current collector (11.7 and 112.1 Ω) for the LFP/GF electrode (It should be ). These results confirm that the GF could not only pointed out that Huang and coworkers (29) recently used a gra- preserve the high conductivity of the overall electrode, but also phene network to improve the rate performance of a LFP cathode.
largely improve the electrochemical activity of LTO during the Different from our preparation process, their LFP/graphene net- work material was prepared by simply mixing a graphene networkwith a suspension of LFP powder by stirring. Moreover, a poly Electrochemical Behavior of the LFP/GF Cathode. To assemble a fullLIB, we also fabricated a LFP/GF cathode, using a similar pro- (vinyl difluoride) binder and acetylene black were used in the cess for LTO/GF fabrication, to match the LTO/GF anode ( fabrication of the LFP/graphene network electrode by magnetic Similar to the LTO/GF anode, this stirring. This LFP/graphene electrode showed a moderate-rate LFP/GF cathode is very flexible and shows excellent cyclic sta- performance similar to LFP/chemically derived graphene electro- bility at both a low rate of 0.5 C and a high rate of 10 C ( des (27, 28), possibly because the graphene conductive network ), which further proves the impor- may be broken during the stirring process, resulting in damage to tance of the 3D conductive interconnected GF network, as well the unique structure and degradation in the properties of the GFs.
as the good contact and strong binding between LFP and the GF.
It is worth pointing out that the charge/discharge curve of the The specific capacity of the LFP/GF electrode at a 50-C dis- pristine GF () shows a long, flat potential charge rate is 98 mAh/g which is higher plateau at about 0.2 V, which means that lithium ions can V vs.
l (
ia 1.5
Capacity (mAh/g) Coulombic Efficiency Characterization of a thin, lightweight, and flexible LTO/GF//LFP/GF full battery. (A) Photograph of a bent battery encapsulated by PDMS, showing its good flexibility. (B) Lighting a red LED device under bending. (C) Galvanostatic charging/discharging curves of the battery. Red and blue lines represent the as-fabricated flat battery and the bent battery after repeatedly bending to a radius of 5 mm 20 times, respectively. (D) Cyclic performance of the battery under flat and bent states.
PNAS October 23, 2012 vol. 109 no. 43 17363 V vs. Li( 1.5
Capacity (mAh/g) Capacity (mAh/g) Rate and cyclic performance of a flexible LTO/GF//LFP/GF full battery. (A) Charging/discharging voltage curves of the battery with different current rates. (B) Capacity of the battery charged/discharged at a constant 10-C rate for 100 cycles.
intercalate into GF below 0.2 V. However, the voltage windows battery being bent decreased less than 1% compared with that of used for the LTO/GF and LFP/GF in this work are 0.8–2.5 V and the original flat battery (Fig. 4C). Moreover, the flexible battery 2.5–4.2 V, respectively. Therefore, these high-voltage windows showed an excellent cyclic stability both under flat and bent prevent lithium intercalation in GF.
states. For example, with respect to the original capacity, the flexible battery showed a capacity retention of ∼97% after the Assembling and Electrochemical Behavior of a Flexible Full Battery.
first 15 cycles under a flat state, and ∼95% after another 15 The voltage profiles of the LTO/GF and LFP/GF electrodes cycles under a bent state (a bend radius of 5 mm) (Fig. 4D).
were investigated before assembling a full battery The high-rate performance of the flexible battery was further and no significant overpotential was observed. The investigated. Fig. 5 shows the insertion/extraction capacity of LTO/GF and LFP/GF electrodes showed initial discharge ca- a flexible battery tested at different charge/discharge rates. It pacities of 170 mAh/g and 164 mAh/g at 0.2 C, respectively.
should be noted that the rate capability of a LTO/LFP full bat- These results indicate that the LTO/GF is a good match with the tery is limited by the cathode material, and it is lower than that of LFP/GF for assembling a full battery. Using the flexible LFP/GF a half cell ). This flexible full battery can cathode and LTO/GF anode, we then built a thin, lightweight, be operated at a 10-C rate with a capacity of 117 mAh/g (Fig.
and flexible LTO/GF//LFP/GF full battery. The free-standing 5A), 88% of the capacity at a 1-C rate, which surpasses most full LTO/GF and LFP/GF electrodes with a thickness of ∼100 μm batteries reported (30–32) despite the fact that they are all based were first laminated onto both sides of a polypropylene separator on a conventional, nonflexible electrode package integrated with and then sealed with ∼250-μm-thick poly (dimethyl siloxane) metal foil current collectors, carbon black additive, and binder, (PDMS) in an Ar-filled glove box using LiPF6 in ethylene car- because the fast charge/discharge performance of a flexible full bonate/dimethyl carbonate as the electrolyte. The total thickness battery has never been reported (6, 11, 13). Furthermore, our of this flexible full battery is less than 800 μm. Due to the small flexible full battery can be cycled over 100 cycles at a high rate of thickness and great flexibility of the GF-based electrodes (Fig.
10 C with only 4% capacity loss (Fig. 5B). Such a high-rate, long- 2A), the full battery shows excellent flexibility, and no structural life performance for this flexible LTO/GF//LFP/GF battery, to failure was observed after repeatedly bending to a radius of <5 mm our knowledge, has never been reached before.
(Fig. 4A). This flexible battery is able to power a red light-emittingdiode (LED) when bent, as shown in Fig. 4B. As expected from the operating voltages of the LTO/GF and LFP/GF, their com- In this work, we have demonstrated a thin, lightweight, and bination produces a battery with an operating voltage of 1.9 V, and flexible LIB using a 3D flexible and conductive interconnected the initial discharge capacity of the battery is ∼143 mAh/g with a GF network as both a highly conductive pathway for electrons/ Coulombic efficiency of 98% at a 0.2-C rate (Fig. 4C). In this full lithium ions and light current collector. By using the flexible battery, all of the inactive components (metal current collectors, LTO/GF and LFP/GF as anode and cathode, respectively, a conducting additives, and binders) are replaced by lightweight GF flexible full battery was assembled. This battery has shown good (nearly two orders of magnitude lighter in areal density and three flexibility, high capacity, high rate, and long-life cyclic perfor- orders of magnitude lighter in volume density than copper); our mance even under repeated bending to a small radius of 5 mm.
flexible battery shows an energy density of ∼110 Wh/kg based Our strategy is versatile and can be used to fabricate a broad on the total mass of the LTO/GF anode and LFP/GF cathode. A class of anode and cathode materials. Both the fabrication of GF higher energy density can be obtained by using materials with and subsequent filling and loading of active materials can be a wider voltage window. Although the volume energy density of easily scaled up, which opens up the possibility for large-scale the LTO/GF//LFP/GF battery is not very high at low discharge fabrication of flexible batteries with high capacity to power rates, it becomes very good at high discharge rates compared with flexible electronic devices that can be operated at a high power the batteries not using GF, and it is possible to increase its energy rate and fully charged in a very short time.
density. For example, the volume energy density of the battery canbe increased by increasing the thickness of active materials, im- proving the assembly process, selecting better packaging materials, Synthesis of LTO/GF. The GF was prepared as previously reported (19). LTO/GF and controlling the thickness of the GF used. To further increase was prepared as follows. Typically, 1.7 mL of 30% (wt/wt) hydrogen peroxide the volume energy density of a flexible battery, one can deposit was dispersed in 40 mL of 0.4 M LiOH, followed by the addition of 3 mmol oftitanium tetraisopropoxide [Ti-(OC electrochemically active materials with extremely high energy 3H7)4]. After stirring for 1 h, a piece of GF was soaked in the solution, which was then transferred to an 80-mL Teflon- density, such as metal oxides and silicon.
lined stainless autoclave and held at 130 °C for 3–12 h before cooling to We also investigated the effect of bending on the performance room temperature. The GF loaded with LTO nanosheets was washed with of the flexible battery. After 20 bends to a radius of 5 mm, only deionized water and dried in an oven at 80 °C. Finally, the as-prepared a negligible overpotential was observed, and the capacity of the sample was heated in a muffle furnace at 550 °C for 6 h in argon. Reference LTO powder was prepared under the same conditions but without the ad- as the counter electrode, 1 M LiPF6 in ethylene carbonate and dimethyl dition of GF to the reaction solution.
carbonate (1:1, vol/vol) as electrolyte, and Celgard 2400 polypropylene asseparator. The charge/discharge cycles were performed at different rates at Synthesis of LFP/GF. In a typical synthesis process (33), 0.01 mol of lithium room temperature. All tests on the free-standing LTO/GF and LFP/GF were acetate hydrate (CH3COOLi·2H2O), iron(III) nitrate hydrate (Fe(NO3)30·9H2O), performed without conventional metal current collectors, carbon black and and ammonium dihydrogen phosphate (NH4H2PO4) were dissolved in 35 mL binder. The reference LTO electrode was prepared by mixing the pure LTO of distilled water; 2.5 mL of ethylene glycol was then added to the solution, powder, carbon black (TIMCAL Super-P), and poly(vinyl difluoride) at and a yellow suspension was obtained. A piece of GF was soaked in this a weight ratio of 80:10:10. The mixture was pasted onto a pure aluminum solution before being transferred to an 80-mL Teflon-lined autoclave. The foil and then pressed and dried under vacuum at 120 °C for 12 h. Batteries autoclave was sealed, kept at 180 °C for 6 h, and then cooled to room tem-perature. The GF covered with LFP nanoparticles was washed with deionized were assembled in an argon-filled glove box with oxygen and water con- water and dried in an oven at 80 °C. Finally, the as-prepared sample was tents below 1 and 0.1 ppm, respectively. All of the capacities and C-rate heated at 720 °C for 12 h under a hydrogen and argon (5:95 vol/vol) atmo- currents in this work were calculated based on LTO and LFP active materials (1 C corresponding to 175 mA/g and 170 mA/g for LTO and LFP, respectively).
The graphene content in LTO/GF and LFP/GF hybrid materials was esti- For the full battery, the capacities and C-rate currents were calculated based mated to be ∼12 wt%.
on the cathode active material (1 C corresponding to 145 mA/g).
PDMS gel was fabricated by vigorously mixing base/curing agents (Sylgard Characterization. XRD patterns of samples were recorded on a Rigaku dif- 184; Dow Corning), followed by degassing in a vacuum oven for 30 min and fractometer using Cu Kα irradiation. SEM and TEM images were obtained on thermally curing at 80 °C for 4 h.
a FEI Nova NanoSEM 430 and Tecnai F20, respectively. Raman measurementswere performed on Jobin-Yvon LabRam HR 800 excited by a 632.8-nm laser.
ACKNOWLEDGMENTS. This work was supported by Ministry of Science andTechnology of China Grant 2012AA030303, National Natural Science Foun- Electrochemical Measurements. For half cell tests of free-standing LTO/GF and dation of China Grants 51172240, 50921004, and 50972147, and Chinese LFP/GF, coin cells were fabricated. In both cases, a lithium metal foil was used Academy of Sciences Grant KGZD-EW-303.
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