7Li NMR studies of lithium ion dynamics in polymer-derived silicon oxycarbide ceramics

Abstract

7Li NMR spin–lattice relaxation, line-shape, and stimulated-echo analyses are performed to investigate lithium ionic motion in a polymer-derived silicon oxycarbide (SiOC) ceramic. In particular, we compare microscopic jump dynamics in lithiated and delithiated compounds. We find that the large variety of lithium environments in the studied SiOC ceramic results in broad distributions of correlation times for the jump motion of the lithium ions, but the experimental results yield no evidence that clearly distinguishable lithium dynamics can be attributed to various electrochemically active lithium sites in different regions of structurally heterogeneous carbon-rich materials. Moreover, we observe that the lithium ionic jumps are, on average, faster in the lithiated than in the delithiated compound at ambient temperatures. Specifically, the mean logarithmic correlation times amount to τm=2⋅10-4s and τm=1⋅10-2s for the former and latter samples at 300K, respectively. On the other hand, the temperature dependence of lithium jumps is higher for lithiated than for delithiated SiOC so that the difference in the ionic mobilities is reduced upon cooling. Finally, we show that the prepared lithiated and delithiated samples undergo irreversible changes when heating to 400K. Specifically, when subject to thermal treatment, 7Li spin–lattice relaxation becomes slower for one lithium fraction, while it remains unaffected for another lithium fraction.