The electrochemistry of nickel in a lithium-based solid polymer electrolyte in ultrahigh vacuum environments Lin-Feng Li a, b , Dana Totir a, b , Yosef Gofer a, b , Gary S. Chottiner b, c , Daniel A. Scherson a, b, * a Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106-7078, USA b Ernest B. Yeager Center for Electrochemical Sciences, Case Western Reserve University, Cleveland, OH 44106-7078, USA c Department of Physics, Case Western Reserve University, Cleveland, OH 44106-7078, USA Received 17 October 1997 Abstract The underpotential deposition (UPD) of lithium on polycrystalline Ni and Ni(111) from LiClO 4 /poly(ethylene)- oxide (PEO) and LiI/PEO electrolytes was examined by cyclic voltammetry in ultrahigh vacuum (UHV) at temperatures, T, in the range 330±340 K. At least two well-de®ned UPD peaks (A and B), and their corresponding stripping counterparts (A' and B'), were identi®ed in the region 0.25±2.0 V vs. Li[C/R]. Their combined charge, Q A+B (or Q A' +B' ), estimated from the smoother Ni(111) specimen, was about 40 mC/cm 2 , i.e. equivalent to a Li coverage (y Li ) of ca. 0.15, assuming Li + undergoes full discharge. The presence of more than a single Li UPD voltammetric feature is consistent with low energy electron diraction (LEED) studies of K, Cs and Li adsorbed on Ni(111), which revealed dierent surface superstructures as a function of the alkali metal coverage (y alk ) for 300 < T < 350 K. Furthermore, the small values of y Li found just prior to bulk Li electrodeposition, are in harmony with (i) additional LEED information, which indicates that a second alkali metal layer begins to form for y alk R 0.5 and (ii) the rapid decrease in the work function of Ni, F Ni (and other high work function metals) as a function of y alk to values lower than F alk for y alk <0.3. Electrodeposition of bulk Li on Ni displayed a nucleation/ growth loop and a sharp stripping peak with no evidence for alloy formation. Marked changes in the voltammetric features could be observed after dosing polycrystalline Ni surfaces with carbon, and especially oxygen, supporting the view that peaks A and B (and A' and B') can indeed be ascribed to Li UPD (and stripping) and not to eects associated with super®cial impurities. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Lithium; Nickel; Solid polymer electrolyte; UHV; UPD; LEED 1. Introduction Nickel may be regarded as a unique electrode ma- terial for the study of lithium electrochemistry, as it is among the very few metals with which Li does not form alloys even at moderately high temperatures, ca. 450 K [1, 2]. This property makes Ni an ideal substrate for exploring fundamental aspects of Li electrodeposi- tion without complications derived from the formation of other phases. Of direct relevance to this research area are studies of alkali metal adsorption on foreign substrates in ultrahigh vacuum (UHV), from which detailed information can be obtained regarding elec- tronic and structural aspects of such interfaces and their chemical reactivity toward other species. Although this general ®eld has been the subject of extensive investigations over the past two decades [3, 4], the adsorption of lithium on Ni has received surpris- ingly little attention [5, 6]. Particularly noteworthy is the work of Chandavarkar et al. [5], who detected rings in the low energy electron diraction (LEED) Electrochimica Acta 44 (1998) 949±955 0013-4686/98/$ - see front matter # 1998 Elsevier Science Ltd. All rights reserved. PII: S0013-4686(98)00198-4 PERGAMON * Corresponding author.