Li dynamics in carbon-rich polymer-derived SiCN ceramics probed by NMR

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.