Nano-encapsulation of graphite-based anodes by a novel polymer electrolyte and its influence on C-rate performances of Li-ion batteries Seok Koo Kim a , Byeong-Jin Shin a , Jong Hun Kim a , Soonho Ahn a , Sang-Young Lee b, * a Batteries R&D, LG Chem, Yusong-gu, Daejon 305-380, Republic of Korea b Department of Chemical Engineering, College of Engineering, Kangwon National University, 192-1, Hyoja 2-dong, Chuncheon, Kangwondo 200-701, Republic of Korea article info Article history: Received 4 August 2008 Received in revised form 13 August 2008 Accepted 14 August 2008 Available online 23 August 2008 Keywords: Lithium-ion battery Graphite anode C-rate performance Safety Polymer electrolyte Nano-encapsulation abstract A unique approach for improving the C-rate (charge/discharge) performances of lithium-ion batteries has been presented, which is based on the nano-encapsulation of graphite (MCMB, mesophase microbead) anodes by the cPVA (cyanoethyl polyvinylalcohol)-gel polymer electrolyte. Through this new process, the gel polymer electrolyte can effectively cover the MCMB surface at nanometer-scaled thickness. The novel morphology of the cPVA coating layer and its high polar –CN groups are considered as key factors to modify the MCMB surface to be electrolyte-philic. This increased polarity of MCMB is expected to allow the favorable impregnation of liquid electrolytes into the pores of the anode and finally contribute to the superior ionic conduction at the faster charge/discharge rates. In addition, it has been demonstrated that the nano-encapsulation of MCMB effectively suppressed the lithium-metal dendrite growth on the charged anode. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Recently lithium-ion rechargeable batteries are expanding to new applications such as hybrid electric vehicles and power tools demanding the high-power density. One of the research activities to achieve this goal is the development of a new anode capable of offering the high-energy density at faster charge/discharge rates, i.e. the excellent C-rate performances. Graphite is considered to be an attractive candidate for an anode active material because it exhibits high electronic conductivities and high reversible capac- ities. However, it has been known that its intrinsic hydrophobicity could cause liquid electrolyte wetting problems in the anodes, which tends to become more significant when especially high- power applications are targeted [1,2]. In addition, this poor wet- ting problem of the anode has been considered to provoke serious concerns about the safeties of lithium-ion batteries [3]. For in- stance, the graphite anode intercalates lithium under the high C-rate charge pulses the anode is polarized to the extent that lith- ium-metal is deposited on the surface of anode. If the thermal management system is inadequate, the lithium-metal creates a potentially catastrophic situation by reacting with both liquid electrolytes and oxygen. Therefore, tackling the poor wetting prob- lems of carbon-based anodes is one of the most crucial issues for improving the C-rate performances and additionally mitigating the accompanied safety concerns. Up to now, most attention for overcoming these challenges has been limited to the size control of electrode active materials [4,5] and the modification of liquid electrolytes [6,7]. We have been much interested in the modification of electrodes by gel polymer electrolytes with well-designed functional groups [8]. In this study, a novel polymer with high polarity, cPVA is em- ployed as a coating material for the purpose of modifying the hydrophobicity of graphite. The presence of –CN groups enables the cPVA to present high dielectric constant (e = 15 at 1 kHz/ 20 °C) that is believed to enhance Li-salt dissociation when it is plasticized with liquid electrolytes, which leads to the superior io- nic conductivity (around 7 mS/cm at 25 °C). This value is remark- ably high, considering the previous publications [9–11] where the ionic conductivities of conventional gel polymer electrolytes have been reported to be around 1 mS/cm. Meanwhile, in a bid to maximize the effect of introducing the gel polymer electrolyte onto the surface of graphite (here, MCMB is employed), a unique approach is suggested, which comprises of, as a first step, the prep- aration of MCMB anode and then the subsequent treatment of it by the cPVA-based gel polymer electrolyte. Through this new process, it is expected that the gel polymer electrolyte can locate effectively onto the MCMB at a well-controlled thickness, with the overall porous structure of anode being preserved. The influence of this surface modification on the morphology and the polarity of MCMB anodes is investigated and compared with those of the pristine MCMB anodes. Finally, the C-rate performances and the lithium- metal dendrite growth onto the charged anodes are also discussed in terms of the polarity change. 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.08.034 * Corresponding author. Tel.: +82 33 250 6338; fax: +82 33 251 3658. E-mail address: syleek@kangwon.ac.kr (S.-Y. Lee). Electrochemistry Communications 10 (2008) 1625–1628 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom