Molecular and Ionic Dynamics in Na x Li 6−x C 60 Nicola Sarzi Amade ̀ , † Daniele Pontiroli,* ,‡ Luca Maidich, † Mauro Riccò , ‡ Mattia Gaboardi, ‡,∥ Giacomo Magnani, ‡ Pietro Carretta, † and Samuele Sanna § † Dipartimento di Fisica, Universita ̀ di Pavia, Via Bassi, 6, 27100 Pavia, Italy ‡ Dipartimento di Scienze Matematiche, Fisiche ed Informatiche, Universita ̀ di Parma, Viale delle Scienze, 7/a, 43124 Parma, Italy § Department of Physics and Astronomy, University of Bologna, Viale C. Berti Pichat 6/2, 40127 Bologna, Italy ∥ ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom ABSTRACT: We report on the C 60 , Na, and Li dynamics in Na x Li 6−x C 60 fullerides (x = 0, 1, 5, and 6) in the temperature range 80−550 K by using 13 C, 23 Na, and 7 Li solid state NMR. The results show that the C 60 reorientation dynamics is hindered at room temperature for the Li-enriched fullerides, but it is active for the Na rich ones with a rate of the order of few kilohertz. 23 Na and 7 Li NMR measurements show the presence of two dominant thermally activated dynamics that can be associated with Li/Na ionic motions within the octahedral sites (intrasite motion) and between the octahedral and tetrahedral sites (intersite motion). The substitution of one Na or one Li ion in the end members Li 6 C 60 and Na 6 C 60 , respectively, yields to an increase of the hopping rate of the intersite motion, which is necessary for the ionic diffusion in possible fulleride-based ionic conductors. ■ INTRODUCTION Lithium and sodium cluster-intercalated fullerides (A x C 60 ;A= Li, Na and x = 6, 12) represent a particular class of alkali fullerides, displaying good performances as hydrogen absorbing materials or as potential components in ion batteries. 1−3 Li and Na ions, thanks to their small ionic radius, are easily intercalated in the interstices of the fullerite lattice with two tetrahedral and one octahedral sites per C 60 molecule. The crystal structure preserves a face-centered cubic (fcc) symmetry (unlike other A 6 C 60 , A = K, Rb, Cs, which are body-centered cubic), thanks to the formation of small alkali clusters. 4−9 These clusters seem to play a fundamental role in the hydrogen absorption process, because they allow the H 2 dissociation and the subsequent migration of the H atoms on C 60 in a reversible way; this mechanism is characterized by significantly faster kinetics and lower hydrogenation temper- ature than pure C 60 . 8,10−12 For instance, it has been reported that Li 6 C 60 , Li 12 C 60 , Na 6 C 60 , and Na 10 C 60 can reversibly absorb up to 5, 4.5, 2, and 3.5 wt % H 2 at T = 200−300 °C, respectively, 10,11,13,14 which can be further increased by addition of catalysts such as Pt/Pd nanoparticles. 15 Moreover, these systems also demonstrated to behave as solid absorbers of indirect hydrogen carriers: for example, Li 6 C 60 can reversibly absorb up to 31.2 wt % of ammonia (corresponding to 5.5 wt % hydrogen) at mild conditions. 16 Recently, the combination of Na and Li cointercalated in the Na x Li 6−x C 60 phases (with x = 0−6) has shown an optimized capacity and kinetics. 17 In particular, NaLi 5 C 60 has proved to absorb up to 4.3 wt % H 2 with an onset temperature 70 °C lower than Li 6 C 60 with the kinetics improved by about 70% and a H 2 desorption enthalpy below 43 kJ/mol H 2 . Furthermore, light alkali and alkali earth (Li, Na, Mg) intercalated fullerides have been received great attention in the recent past, thanks to the ability of the metal ions to diffuse among the C 60 lattice interstices already at low temperature, thus displaying fast ion conductivity. In particular, Li 4 C 60 , thanks to a peculiar polymeric arrangement of the fullerene units, 18,19 displayed a high Li-ion conductivity of 10 −2 S/cm at room temperature (a value comparable to that observed in liquid electrolytes) with a relatively low activation energy. 20 Similarly, large Mg-ion conductivity was also observed in the Mg 2 C 60 compound, which is isostructural to Li 4 C 60 . 21 More recently, a detailed NMR and direct current/alternating current (dc/ac) conductivity study on the alkali-cluster intercalated Li 6 C 60 evidenced the presence of room-temperature Li interdiffusive dynamics also in absence of fullerene polymer- ization, which is hampered upon the hydrogenation of the sample. 22 Some evidence of Na-ions intersite diffusion have been given, either in cubic Na 2 C 60 23 or in the sodium-cluster intercalated Na 6 C 60 , 24 at relatively higher temperature (above T = 400 K). These findings support the possible applications of this class of compounds as solid-state electrolytes in novel ionic batteries. 25−27 In this work, we report a thorough investigation of selected alkali-cluster intercalated fullerides, Na x Li 6−x C 60 with x = 0, 1, 5 Received: January 27, 2017 Revised: March 3, 2017 Published: March 6, 2017 Article pubs.acs.org/JPCC © 2017 American Chemical Society 6554 DOI: 10.1021/acs.jpcc.7b00887 J. Phys. Chem. C 2017, 121, 6554−6560 This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited. Downloaded via 3.236.55.199 on June 13, 2020 at 13:45:57 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.