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 Lil/PEO electrolytes was examined 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 QA_{′ + B′}), estimated from the smoother Ni(111) specimen, was about 40 μC/cm², i.e. equivalent to a Li coverage (θ_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 diffraction (LEED) studies of K, Cs and Li adsorbed on Ni(111), which revealed different surface superstructures as a function of the alkali metal coverage (θ_alk) for 300 < T < 350 K. Furthermore, the small values of θ_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 θ_alk≤0.5 and (ii) the rapid decrease in the work function of Ni, Φ_Ni (and other high work function metals) as a function of θ_alk to values lower than Φ_alk for θ_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 effects associated with superficial impurities.