Back to Table of Contents

46061-AC10
NMR and Structural Investigations of Novel Oxysulfide Intercalation Materials

Clare P. Grey, State University of New York at Stony Brook

A novel class of layered oxysulfides with perovskite-type structure has recently been synthesized, Sr₂MnO₂Cu_2m-0.5S_m+1 (m=1, 2 and 3), which consist of alternating perovskite-type [Sr₂MnO₂] and antifluorite-type [Cu₂S₂] layers of varying thickness. [1-2] Previous work has revealed that lithium can be electrochemically inserted in these compounds. [3] This process is accompanied by an extrusion of Cu metal and has shown to be reversible. The performance of the compounds in lithium batteries is good, but dependent on the thickness of the antifluorite-type layers: as it increases, the initial capacity values are higher, but more pronounced capacity decay is observed. This implies that the presence of the inactive perovskite-type Sr₂MnO₂ layer seems to provide structural stability to the framework and is beneficial to their good electrochemical performance. To develop a deeper understanding of how the structure changes during these lithium de/intercalation processes, the m=2 member, Sr₂MnO₂Cu_3.5S₃ (denoted as MnCuII), was chosen as a model compound and a combination of techniques was used to follow the structural changes in its first discharge/charge processes. More specifically, in situ X-ray Diffraction (XRD), which is sensitive to the average atomic positions, was performed to follow the crystallographic changes, ex situ X-ran Absorption Near Edge Spectroscopies (XANES) on both metal (Cu and Mn) and non-metal (S) elements were used to examine the variation of oxidation states of relevant ions, and Li NMR, which has been proved to be a powerful tool to investigate lithium structure and dynamics in de/intercalation processes, was carried out to probe the local environment changes.

The discharge curve of a battery assembled using MnCuII as the positive electrode and Li metal as the negative electrode from its open circuit voltage (around 3 V) down to 1.1 V shows a long plateau at ca. 1.6 V with up to 4 Li being inserted in the structure. Upon charging to 3.75 V, three processes, at ca. 1.8, 2.4 and 3.3V, were found. Cycling experiments show that the second and following discharge profiles are dependent on which cutoff voltage (2.75 vs. 3.75V) was chosen in the first charge, implying that the process occurring after 2.75 V, e.g. the 3.3 V one, induces structural changes of the host framework. To investigate the structural changes, a specially designed cell [4] for in situ XRD measurements was assembled using MnCuII and Li metal as positive and negative electrodes respectively, and was mounted on a synchrotron beamline. The diffraction data were taken when the battery was cycling between 1.1 and 2.75 V. The diffraction pattern during the 1^st discharge process does not change significantly except that a 2^nd phase appears after 2.0 Li is inserted. Moreover, reflections corresponding to copper metal start to emerge and become more intense after around 1.0 Li is intercalated. Upon charge, the intensity of the copper metal reflections is reduced, but is not zero when the battery was charged to 2.75 V, indicating that not all of the copper returns to the oxysulfide at this voltage. Electrochemically controlled lithiated samples with different lithium contents (Lix with x varying from 1.0 to 4.0, indicating the amount of Li being inserted or remaining in the structure) were also prepared and subjected to ex situ XANES and Li NMR measurements. In the 1^st discharge, the metal K-edge XANES results on both Cu and Mn show shifts to lower energy of the absorption edge positions, which correspond to different oxidation states, indicating the reduction of Cu and Mn in the discharge process. The edge shift for Cu is more significant in samples after Li1.0 than before, while for Mn, most of the reduction occurs before Li2.0. This indicates that Mn is reduced first upon lithium intercalation and Cu is then reduced and extruded as copper metal; this consistent with the in situ XRD results. Formation of a Li₂S-like structure is seen in the S XANES results. The ex situ ⁷Li MAS NMR spectra of the discharged samples show a group of resonances between 200 and 260 ppm region, resulting from the Fermi contact interaction between paramagenetic Mn and Li through bonds with S. The intensities increase upon discharge according to the amount of Li inserted into the structure, indicating that lithium ions are intercalated into chemically different local environments. The change of the line shape and resonance positions implies that the reduction of Mn clearly affects the Li environment. Upon charge, most lithium ions are already extracted when the battery reaches 2.75V, and the higher voltage processes are related to the oxidation of Cu.

References:

[1] N. Barrier et al., Chem. Commun., (2003) 164-165.

[2] Z. A. G‡l et al., J. Am. Chem. Soc., 128 (2006) 8530.

[3] S. Indris et al., J. Am. Chem. Soc., 128 (2006) 13354.

[4] M. Morcrette et al., Electrochimica Acta., 47 (2002), 3137.