Investigation of Li and H dynamics in Li6C60 and Li6C60H

Abstract

We report the extensive investigation of Li and H dynamics in Li 6 C 60 and Li 6 C 60 H y , by combining 7 Li and 1 H solid state NMR measurements with DC/AC conductivity, in order to evaluate the potential application of these systems for energy-storage purposes. 7 Li NMR results show a local motion of Li ions above 200 K in both pristine and hydrogenated compounds, with activation energies of 90-150 meV and correlation times of about 30 ps. Evidences of Li interdiffusive dynamics are given by conductivity measurements in Li 6 C 60 already above 120 K, with activation energies of 240 meV, suggesting that ionic conductivity is of the order of 10 −5 S•cm −1 at room temperature, with correlation times of about 150 ps. On the other hand, the Li 6 C 60 H y behaves like a semiconductor with a high energy gap (ca. 2.5 eV), suggesting that diffusion of intercalated Li ions is prevented. 1 H NMR measurements indicate the absence of H motions for the whole temperature range investigated (up to 360 K), neither on macroscopic or local scale. Li 6 C 60 good properties for H 2-storage are confirmed in terms of absorption capacity (5 wt% H 2), moreover we found that around 35% of lithium segregates in LiH form, leaving Li 4 C 60 H 40 as the final hydrogenation product.

In Situ Neutron Powder Diffraction of Li6C60 for Hydrogen Storage

“…The last step is the slowest because, when LiD reacts with the hydrofulleride (Li 6-α C 60 D 8+y-α ), the limiting process is the ionic diffusion of Li + in the deuterated phase which is reported to be slow. 14 In order to confirm this hypothesis we fit the time evolution of the integrated intensity of the Li 6 The temperature is shown on the right y-axes. 16 The fit is shown in Figure 6 and yields k = 8.4(6) % h -1 , t i = 8.28(1) h, and n = 0.537 (3).…”

H and Li dynamics in Li 12 C 60 and Li 12 C 60 H y

“…The 7 Li 1/T 1 (T) curve in Li 12 C 60 displayed in Figure 7 shows a single peak at T ∼ 400 K. This behavior differs from Li 6 C 60 and Na 6 C 60 [5,29] which shows two peaks, one around 150 K and the other around 400 K. A recent NMR investigation [29] showed that these two peaks reflect two thermally activated ion show a single occupancy [9]. When this structural transition occurs, ionic diffusion through the channel connecting the octahedral and tetrahedral interstices becomes possible.…”

“…Similarly, large Mgion conductivity was also observed in the Mg 2 C 60 compound, isostructural to Li 4 C 60 [25]. More recently, a detailed NMR and DC/AC conductivity study on the alkali-cluster intercalated Li 6 C 60 evidenced the presence of room temperature Li interdiffusive dinamics also in absence of fullerene polymerization, which is hampered upon the hydrogenation of the sample [5]. These findings support the possible applications of this class of compounds as solid-state electrolytes in novel ionic batteries [6,26,27,4].…”

“…Lithium cluster-intercalated fullerides (Li x C 60 ; x = 6 − 12) display good performances as hydrogen absorbing materials or as potential components in ion batteries [1,2,3,4,5,6]. In these systems, the alkali ions usually occupy the tetrahedral and octahedral interstices of the face centered cubic (fcc) C 60 lattice, forming small alkali clusters [7,8,9,10,11,12] that seem to play a fundamental role in the hydrogen absorption process, since they facilitate the H 2 dissociation.…”

On enhanced hydrogen adsorption on alkali (cesium) doped C60 and effects of the quantum nature of the H2 molecule on physisorption energies

“…Moreover, the temperature of the onset of desorption was reduced to about 250 -300 °C, well below the temperature (> 500 °C) at which hydrogen starts to desorb from pure fulleranes [33,34]. Optical spectroscopy, X-ray diffraction, neutron diffraction, NMR and Muon Spin Relaxation have been applied to probe the dynamics and structural changes that accompany ad-and desorption of hydrogen [23,[30][31][32][33][35][36][37][38][39]. Laser-desorption mass spectra of hydrogenated alkali-doped C60 solids show evidence for the presence of HnC60 (n up to 48) fulleranes [23]; other spectra show HnC60 + ions containing as many as 60 hydrogen atoms [32].…”