6Li MAS NMR Investigation of Electrochemical Lithiation of RuO2: Evidence for an Interfacial Storage Mechanism

Nanocrystalline RuO2 was electrochemically lithiated using a Li-6-enriched negative electrode, and selected samples at various states of lithiation-delithiation were characterized ex situ by Li-6 magic-angle spinning nuclear magnetic resonance (Li-6 MAS NMR). In the first plateau (up to one Li per RuO2), a signal with considerable shift and loss of intensity is observed, showing a strongly paramagnetic character for the LiRuO2 phase. A signal due to solid electrolyte interphase (SEI) appears at similar to 0 ppm on this first plateau, but significantly grows only on the subsequent conversion plateau (from 1 to 4 Li/RuO2). Li2O is detected only at the very end of the latter plateau. On further lithiation (4 to 5.5 Li/RuO2), the magnitude of the Li2O signal remains constant, and a new signal at 4 ppm appears, that we can assign to interfacial Li hypothesized earlier in this system. Upon subsequent delithiation, NMR shows that the interfacial Li first disappears, then Li2O also disappears, and the reconstructed Li-RuO2 phase is clearly different from the one formed during the initial lithiation of RuO2. Besides, the SEI signal slightly changes but does not decrease in magnitude upon delithiation. NMR results are in satisfactory agreement with the characteristic features of the proposed "job-sharing" mechanism.