Superionic Conductivity in the ${\mathrm{Li}}_4{\mathbf{C}}_{60}$ Fulleride Polymer

We report on the extraordinary superionic conductivity in the fulleride polymer ${\mathrm{Li}}_4{\mathrm{C}}_{60}$, a crystalline material with no disorder. ${}^7\mathrm{Li}$, NMR, and dc frequency dependent conductivity show uncorrelated ionic hopping across small energy barriers ($\Delta E_a\sim 200\mathrm{meV}$) and an ionic conductivity of ${10}^{-2}\mathrm{S}/\mathrm{cm}$ at room temperature, higher than in “standard” ionic conductors. Ab initio calculations of the molecular structure find intrinsic unoccupied interstitial sites that can be filled by ${\mathrm{Li}}^{+}$ cations in stoichiometric ${\mathrm{Li}}_4{\mathrm{C}}_{60}$ even at low temperatures. The low energy required for the occupation of these sites allows a sizable ${\mathrm{Li}}^{+}$ diffusion above 130 K. The results suggest novel application of lithium intercalated fullerides as electrodes in Li ions batteries.

Figure 1

Temperature dependence of dc conductivity of polymeric ${\mathrm{Li}}_4{\mathrm{C}}_{60}$. The low temperature plateau corresponds to a sample resistance of $20\mathrm{M}\Omega$. Inset: Illustration of the possible paths for ${\mathrm{Li}}^{+}$ ion diffusion in polymeric ${\mathrm{Li}}_4{\mathrm{C}}_{60}$. Blue balls are fullerene molecules, the pink/grey (medium grey/light grey) region is the volume which the centers of diffusing Li ions can occupy (see text).

Figure 2

Upper panel: frequency dependence of the imaginary part of the impedance, $Z^{\prime \prime }$, at different temperatures. The solid line represents the fit with an ideal Debye loss function. Lower panel, left: corresponding Cole-Cole diagrams. Lower panel, right: correlation times extracted from the frequency dependence of $Z^{\prime \prime }$.

Figure 3

Temperature dependence of ${}^7\mathrm{Li}$ NMR relaxation rate in ${\mathrm{Li}}_4{\mathrm{C}}_{60}$ at the field of 2 T (right scale, ${\omega }_0/2\pi =33.10\mathrm{MHz}$) and 5 T (left scale, ${\omega }_0/2\pi =82.75\mathrm{MHz}$).

Figure 4

Occupied [green (light grey) balls] and unoccupied (empty circles) ${\mathrm{Li}}^{+}$ interstitial positions in the octahedral site according to ab initio calculations. These four sites lie on the $b=0$ plane at coordinates $(0.18,0,0.36{)}_{\mathrm{occup}}$ and $(0.13,0,0.64{)}_{\mathrm{unoccup}}$ respectively. ${\mathrm{Li}}^{+}$ ions in the tetrahedral sites are indicated by red (dark grey) balls. The positions of the ${\mathrm{Li}}^{+}$ ions reported in a previous x-ray diffraction study (crosses) [4] are also indicated.