Invited: Investigation of Carbon/Copper Multilayer to Examine the Influence of Copper Coating on the Li-Storage Performance of Carbon

Thin copper and carbon coatings of electrodes of lithium-ion batteries (LIBs) have the potential to improve LIB operation by preserving electrode integrity during cycling, by developing a proper solid-electrolyte interphase (SEI) layer (e.g., by increasing the de-solvation rate), and by enhancing electric conductivity. In the structures, the thin coatings, e.g., copper thin films, must be permeable to Li⁺ ions in order to facilitate Li⁺ uptake and Li⁺ release in the electrochemically active material of coated electrodes beneath. The influences of copper and carbon thin coatings on LIB-electrode performance were investigated in this work by electrochemically cycling a [C(16 nm)/Cu(17 nm)] × 10 multilayer (ML) up to lithium plating. The C/Cu ML was deposited onto a copper current collector using ion beam sputtering. The rate capability and the long-time cycling were compared to the corresponding ones for the cycling of the bare copper substrate and 16 nm and 230 nm carbon single films (without Cu coating). The bare copper electrode does not store Li⁺ ions, which is as expected because copper is electrochemically inactive with respect to lithiation. The Li⁺ uptake and Li⁺ release in thin carbon layers capped by thin copper layers within the C/Cu ML is compared to that of uncapped carbon single thin films. All electrodes exhibited a good rate capability and long-term cycling stability. Under fast cycling, the amount of reversible Li⁺ uptake and Li⁺ release was largest for the case of the C/Cu ML, which pointed to the beneficial influence of the capping Cu layers. The higher Li kinetics in the C/Cu ML was confirmed using impedance analysis. The C/Cu ML behaves as a supercapacitor possessing a differential charge plot nearly independent of potential. At lower currents, the specific capacity of the C/Cu ML is only 20% of that of the thin carbon single films, with that of the latter being the same as that of graphite. On the one hand, this evidences a disadvantageous influence of the thin Cu layers, which block the Li⁺ permeation, that is necessary to reach deeper carbon layers of the C/Cu ML electrode. On the other hand, the differential capacity plots reveal that the carbon material in the interior of the C/Cu ML is electrochemically cycled. Microscopy, Raman scattering, depth profiling with X-ray reflectometry (XRR), and secondary ion mass spectrometry (SIMS) were applied to get deep insights and a comprehensive examination of the contradiction. The XRR examination revealed a non-altered ML after more than 542 electrochemical cycles, after the washing procedure, and even after 15 months of air exposure. This observation suggests that the copper layers block contamination as well as the Li insertion. The analyses of microscopy, Raman, and SIMS affirm the ML intactness but also reveal the participation of some portions of the interior of the C/Cu ML in electrochemical cycling. The low capacity of carbon in the C/Cu ML may stem from the mechanical stress inside the C/Cu ML, which reduces the Li⁺ uptake and Li⁺ release.