In Situ Morphology Observations of Electrodeposited Lithium in Room-Temperature IonicLiquids by Optical Microscopy Hikaru Sano,* 1,2 Hikari Sakaebe, 1,2 and Hajime Matsumoto 1,2 1 National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577 2 CREST, Japan Science and Technology Agency, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577 (Received October 15, 2012; CL-121054; E-mail:hikaru.sano@aist.go.jp) The morphology of electrodeposited lithium in room- temperature ionic liquids (RTILs) was observed by insitu optical microscopy. The dendrite suppression effect was partly confirmed for a PP13[FSA]-based electrolyte containing vinyl- ene carbonate (VC). The factors for dendrite suppression are discussed; stability under cathodic conditions such as E Li=Li þ and viscosity of the base electrolyte are considered to be the relevant factors. Lithium (Li) metal has the most negative redox potential and the lowest density of all metals. Li metalis therefore one of the most attractive anode materials for batteries. However, secondary batteries with a Li metal anode have not yet come into widespread use. One reason is the dendritic growth of Li during charging, which can lead to short circuits and limited life. The morphology of the electrodeposited Li is thought to be determined by the film formed at the interface between Li and the electrolyte. 1 Many attempts have been made to change the surface film properties, but so far they have not borne fruit. 26 A series of room-temperature ionic liquids (RTILs) have attracted much recent attention among electrochemists, as new solvents. 7 RTILs may help to improve battery safety because of their nonflammability and nonvolatility. Some properties of RTILs can be designed, including the cathodic and anodic limit potentials. The surface film formed between a Li metal anode and an RTIL electrolyte is thought to be different from the surface film formed between Li metal and a conventional organicelectrolyte, especially for RTILs that show high stability under very cathodic conditions such as the Li redox potential. As mentioned above, changing the surface film properties by using such RTILs affects battery performance and the cyclability of full cells and half cellswith RTILs and Li metal anodes, as reported by some researchers. 811 However, there have been few reports on the morphology of electrodeposited Liin RTILs. 1214 In our previous study, 15 we reported the morphology of electrodeposited Liin a PP13[TFSA]-based [PP13: N-methyl- N-propylpiperidinium, TFSA: bis(trifluoromethanesulfonyl)- amide] electrolyte and an EMI[FSA]-based [EMI: 1-ethyl-3- methylimidazolium, FSA: bis(fluorosulfonyl)amide] electrolyte, with and without vinylene carbonate (VC). Dendritic growth was suppressed only in the cell using a PP13[TFSA]-based electrolyte with VC; only fine and nondendritic growth of Li was observed. However, the electrolyte properties are too different to specify the factors causing the dendrite suppression; the cation and anion, and thus viscosity and stability under cathodic conditions, are too different. In the present study, to complete the matrix, PP13[FSA] and EMI[TFSA] were selected as base electrolytes, and Li deposition morphologies in these electrolytes were examined to clarify the factors affecting dendrite suppression. In situ optical microscopy observations were mainly used to investigate the morphology of the electro- deposited Li, as it is not affected by the sampling process, including rinsing and transportation. Electrolyte preparation was carried out in accordance with previous reports; 8,11 0.32 mol kg ¹1 (ca.10 wt %)of Li[TFSA] (Kishida Chemical) was added to PP13[FSA] (Dai-ichi Kogyo Seiyaku, Mitsubishi Materials) and EMI[TFSA] (Kanto Chemi- cal), and 0.35 mol kg ¹1 (ca. 3 wt %)of VC was added to each electrolyte. Figure 1 shows the chemical structures of the RTIL cations, anions, and VC. For the insitu optical microscopy observations and ex situ scanning electron microscopy (SEM) observations, custom- made two-electrode cells were fabricated in an argon (Ar)-filled glove box in accordance with previous reports. 15 Nickel (Ni) foil and Li film pasted on copper foil (Honjo metal, battery grade) were used for the working electrode and counter electrode, respectively. The insitu observation of the working electrode surface was carried out, using an optical microscope, through a hole punched in the center of the counter electrode. Li deposition was carried out with a current density of 50 ¯A cm ¹2 for 60000 s. After the electrochemical deposition, the cells were opened in the Ar-filled glove box and the working electrode samples were transferred to a SEM chamber using a custom- made transfer vessel, and the working electrode surfaces were observed by SEM in accordance with previous reports. 15 Figure 1. Structuralformulae of RTIL cations and anions and VC. Published on the web December 29, 2012 77 doi:10.1246/cl.2013.77 © 2013 The Chemical Society of Japan Chem. Lett. 2013, 42, 7779 www.csj.jp/journals/chem-lett/