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

Nanocrystalline RuO 2 was electrochemically lithiated using a 6 Li-enriched negative electrode, and selected samples at various states of lithiation-delithiation were characterized ex situ by 6 Li magic-angle spinning nuclear magnetic resonance ( 6 Li MAS NMR). In the first plateau (up to one Li per RuO 2 ), a signal with considerable shift and loss of intensity is observed, showing a strongly paramagnetic character for the LiRuO 2 phase. A signal due to solid electrolyte interphase (SEI) appears at ∼0 ppm on this first plateau, but significantly grows only on the subsequent conversion plateau (from 1 to 4 Li/RuO 2 ). Li 2 O is detected only at the very end of the latter plateau. On further lithiation (4 to 5.5 Li/RuO 2 ), the magnitude of the Li 2 O 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 Li 2 O also disappears, and the reconstructed Li-RuO 2 phase is clearly different from the one formed during the initial lithiation of RuO 2 . 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.