Thin Flexible Lithium-Ion Battery Featuring Graphite Paper Based

169 ARTICLE Thin flexible lithium-ion battery featuring graphite paper based current collectors with enhanced conductivity Hang Qu, Jingshan Hou, Yufeng Tang, Oleg Semenikhin, and Maksim Skorobogatiy Abstract: A flexible, lightweight, and high conductivity current collector is the key element that enables fabrication of a high-performance, flexible lithium-ion battery. Here, we report a thin, lightweight, and flexible lithium-ion battery that uses graphite papers deposited with nano-sized metallic layers as the current collector, LiFePO4 and Li4Ti5O12 as the cathode and anode materials, respectively, and a PE membrane soaked in LiPF6 as the separator. Using thin, flexible graphite paper as a substrate for the current collector instead of a rigid, heavy metal foil enables us to demonstrate an ultra-thin lithium-ion battery (total thickness including encapsulation layers of less than 250 ␮m) that also features a light weight and high flexibility. Key words: lithium-ion batteries, flexible batteries, graphite paper current collectors. Résumé : Un collecteur de courant a` conductivité élevée, a` la fois flexible et léger, constitue un élément clé pour permettre la fabrication d’une batterie au lithium-ion flexible a` haut rendement. Dans le présent article, nous rapportons la fabrication d’une batterie au lithium-ion mince, flexible et légère constituée de collecteurs de courant faits de feuilles de graphite déposées sur des nanocouches métalliques, d’une électrode composée d’une cathode de LiFePO4 et d’une anode de Li4Ti5O12, et d’un séparateur consistant en une membrane imbibée de LiPF6. L’utilisation d’une feuille de graphite flexible comme substrat du collecteur de courant au lieu d’une feuille métallique lourde et rigide nous permet de démontrer la réalisation d’une batterie au lithium-ion ultramince (épaisseur totale comprenant des couches d’encapsulation de moins de 250 ␮m) qui présente également un faible poids et une grande flexibilité. [Traduit par la Rédaction] Mots-clés : batteries au lithium-ion, batteries flexibles, collecteurs de courant en feuille de graphite. Introduction example, Wang et al. reported a CNT current collector fabricated Many wearable and portable electronic devices require efficient, by cross-stacking continuous CNT films drawn from super-aligned 9 compliant power sources that can fully function when bent, folded, CNT arrays. A flexible LIB electrode could be then deposited on or compressed. Lithium-ion batteries (LIBs) dominate the portable the fabricated CNT films. Compared with metallic current collec- power-source market due to their high energy density, high out- tors, CNT films exhibit a stronger adherence to battery materials. put voltage, long-term stability, and environmentally friendly op- Besides, Li et al. developed flexible nano-porous CNT films directly eration.1 High-performance flexible LIBs are considered to be one on a polypropylene separator using a vacuum filtration technique.10 of the most promising candidates of power sources for the next The as-fabricated CNT films were utilized as binder-free and cur- generation flexible electronic devices.1–3 The LIBs typically consist rent collector-free anodes in LIBs. An electrochemical half-cell was of several functional layers (Fig. 1a). When battery flexibility is fabricated using a CNT film as the anode and a lithium-foil as the desired, all of the battery components should be flexible.1 Among counter electrode; the specific capacity of the CNT film anode was the various functional layers, the current collectors critically af- measured to be ϳ380 mAh/g. Later, the same group also fabricated fect the battery performance, and their flexibility is typically dif- an LIB electrode based on CNT/graphite-nanosheets (GN) composite ficult to achieve together with a high conductivity. films with an optimized CNT/GN ratio of 2:1.11 The reversible ca- Many approaches have been explored for designing flexible pacity of the CNT/GN electrode was found to be 375 mAh/g. More LIBs.4–8 Traditionally, electrode active materials are coated onto a recently, Yoon et al. synthesized CNT films via chemical vapor Cu foil, which works as the anode, and an Al foil is generally used deposition followed by a direct spinning process.12 The CNT films as the cathode. The as-fabricated LIBs are typically heavy and rigid, were then processed by a heating treatment to increase the crys- which makes them unsuitable for truly wearable applications. Re- cently, tremendous effort has been dedicated to the R&D of LIBs talline perfection. Experimental results showed that the electrode that utilize thin, flexible current collectors and free-standing elec- based on heat-treated CNT films exhibited a higher capacity of ϳ trodes. Among these studies, conductive films based on carbon 446 mAh/g that was around twice the capacity of the electrode nanotubes (CNTs) or CNT composites are popularly used as cur- based on raw CNT films. Although CNT films feature great elec- rent collectors or binder-free electrodes due to their appealing trochemical properties for LIB applications, the synthesis of CNT electrical and mechanical properties such as high conductivity, (or CNT-composite) films normally requires a sophisticated pro- high mechanical strength, and large activated surface areas. For cess, and the cost of such materials is high (ϳ1000$ per gram), Received 9 December 2015. Accepted 14 October 2016. H. Qu, J. Hou, and M. Skorobogatiy. Department of Physics Engineering, Ecole Polytechnique de Montreal, Montreal, QC H3C 3A7, Canada. Y. Tang. CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China. O. Semenikhin. Department of Chemistry, Western University, London, ON N6A 5B7, Canada. Corresponding author: Hang Qu (email: [email protected]). Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink. Can. J. Chem. 95: 169–173 (2017) dx.doi.org/10.1139/cjc-2015-0593 Published at www.nrcresearchpress.com/cjc on 24 November 2016. 170 Can. J. Chem. Vol. 95, 2017 Fig. 1. (a) Schematic of the thin, flexible lithium-ion battery. (b) A lithium-ion battery sample. [Colour online.] thus creating significant barriers that prevent the utilization of To synthesize the anode and cathode, LFP and LTO were first LIBs in the wearable devices. pre-mixed with MWCNT in a mortar, respectively. Then, the mixture Flexible LIBs can be also produced by the micro-electromechanical was then dispersed in polyvinylidene fluoride (PVDF)/1-methyl-2- systems (MEMS) fabrication technique in which the battery active pyrrolidone solution using a magnetic stirrer for 4 h. The optimal materials are deposited on a flexible substrate in sequence (e.g., in weight ratio of LFP (or LTO), MWCNT, and PVDF was found to be cathode–electrolyte–anode sequence).13,14 As an example, Su et al. 8:1:1. The obtained slurry was then poured onto the current collector demonstrated a flexible LIB fabricated by a sequential deposition and made into a uniform wet film using a Micrometer Adjustable of a LiMnO2 cathode layer, a LiPON electrolyte layer, and a Li anode Film Applicator (MTI Corporation). The composites were dried in a layer on a 70 ␮m thick stainless steel substrate using the RF- vacuum furnace at 80 °C for 3 h and, then, cut into 2 cm2 ×2cm2 sputtering technique.13 The as-fabricated LIB had a capacity of square-shaped electrodes. These electrodes were then further dried 12.8 ␮Ah when discharged at a current density of 5 ␮A/cm2. Also at 110 °C for 12 h to ensure that the solvent (1-methyl-2-pyrrolidone) based on the MEMS fabrication technique, Vieira et al. reported in the slurry was completely evaporated. fabrication of a flexible LIB that used Ge as anode, LiCoO2 as cathode, The LIBs were finally assembled by stacking the as-prepared elec- and LiPON as solid-state electrolyte.14 During the fabrication pro- trodes together with a PE separator layer. Three droplets (ϳ0.15 mL) cess, Si3N4 and LiPO thin layers were also deposited onto the of electrolyte (1.0 M LiPF6) dissolved in ethylene carbonate/dimethyl cathode and anode, respectively, to provide electrical insulation carbonate (EC/DEC) = 50/50 (v/v) from Sigma-Aldrich were added to and a battery chemical stability safeguard. The battery had a capacity the battery during the stacking process. Note that the battery of ϳ46 nAh/cm2. We note that in a MEMS fabrication process, expen- assembly process was completed in a N2-filled glove box to avoid sive deposition equipment such as RF-sputtering systems are gener- oxidation of the electrolyte. Finally, the battery was encapsulated ally required to have precise control of the coated battery layer. with PE films using a lamination machine before it was taken out In this paper, we report a flexible LIB using graphite paper (GP) for characterization. with enhanced conductivity as current collectors (Fig. 1b). The enhancement of conductivity of GP was achieved by depositing a Characterization sub-micron thick metal layer onto a commercial GP by physical The image of the cross section of the battery sample was taken vapor deposition (PVD). Particularly, we use an Al-deposited GP as by an optical microscope (Fig. 2a). The tomography of the Al coat- the current collector for cathode and a bare GP or a Cu-deposited ing on GP substrate was investigated by scanning electron micros- GP as the current collector for anode. In this LIB, LiFePO4 (LFP) and copy, and an energy dispersive spectrum (EDS) of the Al@GP was Li4Ti5O12 (LTO) are used as cathode and anode active materials, measured (Fig. 2b). The charge–discharge tests and EIS measure- respectively, and a polyethylene (PE) nanostructured membrane ments were performed by an Ivium Stat.XR electrochemical work- is used as a separator.

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