The electrochemistry of nickel in a lithium-based solid polymer electrolyte in ultrahigh vacuum environments

The underpotential deposition (UPD) of lithium on polycrystalline Ni and Ni (111) from LiClO4/poly(ethylene)oxide (PEO) and LiI/PEO electrolytes was ex amined by cyclic voltammetry in, ultrahigh vacuum (UHV) at temperatures, T, in the range 330-340 K. At least two well-defined UPD peaks (A and B), and their corresponding stripping counterparts (A' and B'), were identified 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 mu C/c m(2), i.e. equivalent to a Li coverage (theta(Li)) of ca, 0.15, assuming Li + undergoes full discharge. The presence of more than a single Li UPD volta mmetric feature is consistent with low energy electron diffraction (LEED) s tudies of K, Cs and Li adsorbed on Ni(111), which revealed different surfac e superstructures as a function of the alkali metal coverage (theta(alk)) f or 300 < T < 350 K. Furthermore, the small values of theta(Li) found just p rior to bulk Li electrodeposition, are in harmony with (i) additional LEED information, which indicates that a second alkali metal layer begins to for m for theta(alk)less than or equal to 0.5 and (ii) the rapid decrease in th e work function of Ni, Phi(Ni) (and other high work function metals) as a f unction of theta(alk) to values lower than theta(alk) for theta(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 polycrystallin e 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 land strippi ng) and not to effects associated with superficial impurities. (C) 1998 Els evier Science Ltd. All rights reserved.