Flexible Graphene-Based Lithium Ion Batteries with Ultrafast Charge and Discharge Rates

Flexible graphene-based lithium ion batteries with ultrafast charge and discharge rates Na 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 bDepartment of 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 June 13, 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 vapor deposition-grown graphene can be used as the fast transport he development of next-generation flexible electronics (1), channel of charge carriers. The electrical conductivity of the ∼ Tsuch 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 ∼ × 6 SI Appendix able health-monitoring electronic skin, requires power sources that 1.36 10 S/m ( ). Moreover, the GF is extremely ∼ 2 ∼ μ fl are flexible (2, 3). Similar to conventional energy storage devices, light ( 0.1 mg/cm with a thickness of 100 m) and exible. It ∼ fi flexible power sources with high capacity and rate performance possesses a high porosity of 99.7% and a very high speci c 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 in electrochemical environments. These features give the GF great ium ion batteries (LIBs) have a high capacity but usually suffer fl from a low charge/discharge rate compared with another important potential for use in next-generation exible electronics. Using the electrochemical storage device, supercapacitors. Therefore, it is GF network as both a highly conductive pathway for electrons and 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 (LFP)/GF and system with a supercapacitor-like fast charge/discharge rate and 4 Li Ti O (LTO)/GF electrodes that can simultaneously obtain battery-like high capacity. However, the fabrication of such an 4 5 12 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- fl fl exible bulk electrodes, we further assembled a thin, lightweight, ductivity, robust mechanical exibility, 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 oxide nanowires (8, 9) and carbon nanomaterials such as carbon Results and Discussion nanotubes (6, 10–12) and graphene paper (13) have been re- fl Synthesis and Characterization of Free-Standing Flexible LTO/GF and cently demonstrated for use in exible 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 a consequence, only a moderate charge/discharge rate has been fl obtained in these exible 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: [email protected]. electron transport rates (4, 5, 14, 15), or reducing the path length This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. over which electrons and lithium ions have to move by using 1073/pnas.1210072109/-/DCSupplemental. 17360–17365 | PNAS | October 23, 2012 | vol. 109 | no. 43 www.pnas.org/cgi/doi/10.1073/pnas.1210072109 Downloaded by guest on October 1, 2021 e Cathode Separator Anode e Fig. 1. 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 SI Appendix, Fig. S1). 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 (SI Appendix). 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 (SI Appendix, Fig. S2). materials on GF followed by heating in an argon atmosphere The transmission electron microscope (TEM) image (Fig. 2E) (details in Methods and SI Appendix). 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 (SI Appendix, Fig. S3), which is consistent with the X-ray A B C 300 µm 1 µm D E F ENGINEERING Graphene 200 nm LTO 40 nm Fig. 2. Characterization of a free-standing flexible LTO/GF. (A) Photograph of a free-standing flexible LTO/GF being bent (2 × 2cm2). (B and C) SEM images of the LTO/GF.

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