Li dynamics in carbon-rich polymer-derived SiCN ceramics probed by nuclear magnetic resonance

Abstract

We report ${}^7$Li, ${}^{29}$Si, and ${}^{13}$C NMR studies of two different carbon-rich SiCN ceramics SiCN-1 and SiCN-3 derived from the preceramic polymers polyphenylvinylsilylcarbodiimide and polyphenylvinylsilazane, respectively. From the spectral analysis of the three nuclei at room temperature, we find that only the ${}^{13}$C spectrum is strongly influenced by Li insertion/extraction, suggesting that carbon phases are the major electrochemically active sites for Li storage. Temperature and Larmor frequency (${\omega }_L$) dependences of the ${}^7$Li linewidth and spin-lattice relaxation rates $T_1^{-1}$ are described by an activated law with the activation energy $E_A$ of 0.31 eV and the correlation time ${\tau }_0$ in the high temperature limit of 1.3 ps. The $3/2$ power law dependence of $T_1^{-1}$ on ${\omega }_L$ which deviates from the standard Bloembergen, Purcell, and Pound (BPP) model implies that the Li motion on the $\mu$s timescale is governed by continuum diffusion mechanism rather than jump diffusion. On the other hand, the rotating frame relaxation rate $T_{1\rho }^{-1}$ results suggest that the slow motion of Li on the ms timescale may be affected by complex diffusion and/or non-diffusion processes.

28 pages with double line spacing, 7 figures and 2 tables, accepted version in Journal of Power Sources

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