Suppression of lithium deposition at sub-zero temperatures on graphite by surface modification Nanda Gunawardhana a, ⁎, Nikolay Dimov a , Manickam Sasidharan b , Gum-Jae Park a , Hiroyoshi Nakamura b , Masaki Yoshio a, ⁎ a Advanced Research Center, Saga University, 1341 Yoga-machi, Saga 840-0047, Japan b Department of Chemistry and Applied Chemistry, Saga University, 1 Honjo, Saga 840-8502, Japan abstract article info Article history: Received 2 June 2011 Received in revised form 24 June 2011 Accepted 14 July 2011 Available online 22 July 2011 Keywords: Safety of graphite anode Carbon coating Lithium deposition Pulse polarization Lithium deposition on graphite anodes is considered as a main reason for failures and safety for lithium ion batteries (LIB). Different amounts of carbon coating on the surface of natural graphite are used in this work to suppress the amount of lithium deposited at -10 °C. Pulse polarization experiments reveal relative polarization of graphite anodes at various temperatures and show that lithium deposition is accelerated at lowered temperatures. Electrochemical experiments, along with photographs, scanning electron microscopy (SEM) images and ex-situ X-ray diffraction (XRD) data suggest that carbon coating not only suppresses the lithium deposition but also enhances the formation of LiC 6 at -10 °C. The homogeneous potential profile on the graphite surface attained by the carbon coating explains such an improved low temperature performance, as it allows efficient Solid Electrolyte Interface (SEI) film formation, which is a prerequisite for safety LIB. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Nowadays, lithium ion batteries (LIB) are used in most portable electronic devices and are considered as a power source for plug-in hybrid vehicles (P-HEV) and electric vehicles (EV) [1,2]. However, scaling up the size of LIB increases the chances of accidental explosions. The main reason is lithium dendrite formation on the surface of the graphite anode. The rate of dendrite formation is significantly higher at lowered temperatures and/or higher cycling rates [3,4]. Under such conditions, lithium deposited on the surface of the graphite not only reduces its performance but may also grow until it reaches to the positive electrode. Such short circuits may cause thermal runaways and result in severe accidents. So far, the only solution to this problem is to use anodes with higher working voltage vs. Li/Li + such as lithium titanate. However, such an approach comes at the expense of the energy density of the LIB. Recently, many researchers have focused on enhancing the performance of the graphite anodes at low temperatures. New electrolytes with high ionic conductivities and lower freezing points are under investigation [5]. In addition, various types of additives are employed to form a stable SEI film [6–8]. With this point of view, we have introduced 1–3 propane sultone to enhance the performance of the natural graphite at low temperatures [9]. In our opinion, modification of the graphite itself can also minimize lithium deposition on the graphite surface. Such a modification could be achieved by uniform carbon coating on graphite by chemical vapor deposition (CVD). The aim of this work is to evaluate the carbon coating of natural graphite for suppressing lithium deposition at lowered temperatures. 2. Experimental Carbon coated natural graphite was prepared by the CVD technique described in our previous report [10]. Four samples with a variable amount of carbon coating were used. The samples are denoted NG-0, NG-3, NG-10 and NG-15, where the number implies the amount of the carbon coating (weight percent). Sample NG-0 has no carbon coating and serves as a reference. The charge and discharge characteristics of the graphite electrodes were examined in a screw- type cell, comprised of a lithium metal electrode (Cyprus Foote Mineral Co.) and a graphite electrode separated by two polyamide separators. For the semi-quantitative analysis of XRD data, a titanium mesh was used as a current collector. Mass loading of the electrodes was 5–6 mg.cm -2 . Cyclic voltammograms (CV) were recorded by a Hokuto Denko HSV-100 (Japan) in a beaker type cell, which contains a graphite working electrode, a gold reference electrode and a lithium counter electrode. Other experimental details and instrumentation methods are described elsewhere [9]. 3. Results and discussion The following reasons account for the worsening performance of graphite anodes at low temperatures: 1) reduced conductivity of the Electrochemistry Communications 13 (2011) 1116–1118 ⁎ Corresponding authors. Tel./fax: + 81 952 20 4729. E-mail addresses: kgngu@yahoo.com (N. Gunawardhana), yoshio@cc.saga-u.ac.jp (M. Yoshio). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.07.014 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom