High-entropy
oxides (HEOs), which are a new class of single-phase
solid solution materials, have recently attracted significant attention
as an anode material for lithium-ion batteries (LIBs). In this study,
(MgCoNiZn)1–x
Li
x
O (x = 0.05, 0.15, 0.25, and 0.35) HEOs were
synthesized and their electrochemical performances as the anode material
were observed in LIBs. X-ray photoelectron spectroscopy (XPS) analysis
showed that the increase in the lithium cation concentration causes
generation of more oxygen vacancies, which greatly affected the electrochemical
performance of (MgCoNiZn)1–x
Li
x
O HEO anodes, in the structure. The more
the oxygen vacancy concentration in the anode, the higher the discharge
capacity in the LIB. The (MgCoNiZn)0.65Li0.35O anode had 1930 mA h g–1 initial and 610 mA h
g–1 stable (after 130 cycles) discharge capacities
at a current density of 1000 mA g–1. This work clearly
indicated that designing a HEO with abundant oxygen vacancies in the
structure was a very efficient strategy to improve the electrochemical
performance of the HEO electrode for LIBs.
The widely used engineering material copper is a prototype of an electrochemically passive metal. In this work, the passive films on evaporated copper in 0.1 M NaOH are investigated in situ and operando by spectroscopic ellipsometry and Raman spectroscopy, both conducted during oxidation in potentiostatic step experiments. Oxide growth is initiated by jumping from a potential at which the surface is oxide-free to -0.1 V vs. Ag|AgCl|3 M KCl (+0.11 V vs. standard hydrogen electrode, SHE). At subsequent electrode potential jumps, no corresponding jumps in the thickness are observed; instead, oxide growth proceeds steadily. Above +0.3 V vs. Ag|AgCl|3 M KCl (+0.51 V vs. SHE), the oxide layer thickness remains constant at ≈7 nm. Raman spectra show a peak at 530 cm(-1), which agrees with the dominant peak in spectra of copper mixed oxide, Cu4O3 (Cu2(I)Cu2(II)O3). Crystalline Cu4O3 nucleates from a precursor state showing strong photoluminescence (PL), which hints at the involvement of Cu2O. Overall, the PL spectra of the growing oxide and absorption spectra indicate the presence of Cu2O in the thin films. Absorption spectra cannot be understood as a superposition of the spectra from different well-described copper oxides, which points to defect-rich oxides that show rather different spectra. Raman spectra also point to an involvement of both crystalline and amorphous oxides that coexist. The results show that the passive layers on copper are more complex than the duplex layers described in the literature; they do contain an oxide with a mixed valency of copper.
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