7 Li MAS NMR Studies of Lithiated Manganese Dioxide Tunnel Structures: Pyrolusite and Ramsdellite
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Li MAS NMR Studies of Lithiated Manganese Dioxide Tunnel Structures: Pyrolusite and Ramsdellite
Younkee Paik, Young J. Lee, Francis Wang,1 William Bowden,2 and Clare P. Grey Department of Chemistry, SUNY at Stony Brook, Stony Brook, NY11794-3400, U.S.A. 1 Duracell Global Science Center, Danbury CT 06801 2 Gillette Advanced Technology Center, U.S.A. ABSTRACT The one-dimensional 1×1 and 1×2 tunnel structures of manganese dioxides, pyrolusite (βMnO2) and ramsdellite (R-MnO2), respectively, were chemically intercalated with LiI. Two 7Li resonances were observed in lithiated pyrolusite. One isotropic resonance arising at 110 ppm shows a short spin-lattice relaxation time (T1 ~ 3 ms) and was assigned to Li+ ions in the 1×1 tunnel structure. The other isotropic resonance arising at 4 ppm shows a long spin-lattice relaxation time (T1 ~ 100 ms) and was assigned to Li+ ions in diamagnetic local environments in the form of impurities such as Li2O or on the surface of the MnO2 particles. Three 7Li resonances were observed in lithiated ramsdellite at very different frequencies (600, 110 and 0 ppm). The resonance at 600 ppm, which is observed at low lithium intercalation levels, is assigned to Li+ ions coordinated to both Mn(III) and Mn(IV) ions in the 1×2 tunnels, while the resonance at 110 ppm is due to Li+ ions coordinated to Mn(III) ions and appears at higher Li levels. The resonance at 0 ppm is associated with a long spin-lattice relaxation time (T1 ~ 100 ms) and is also assigned to Li+ ions in diamagnetic impurities. INTRODUCTION Manganese dioxides are used worldwide as cathodes in lithium and zinc primary batteries [1]. These cathodes contain the highly disordered manganese dioxide EMD (electrolytic manganese dioxide), which consists of an intergrowth of the pyrolusite and ramsdellite structures [2]. In order to understand the mechanism of intercalation in this complex material, our work has focussed on understanding the intercalation processes that occur in the model systems pyrolusite (β-MnO2) and ramsdellite (R-MnO2). 7Li MAS NMR spectroscopy has been used to study these systems since it represents an ideal method for probing the local atomic and electronic environments of the lithium ions [3]. EXPERIMENTAL DETAILS Pyrolusite (β-MnO2) was purchased from Fisher Scientific and ramsdellite (R-MnO2) was purchased from Erachem. The host compounds were well ground in a mortar and dried under vacuum at 60 oC for 30 hours (β-MnO2) or at 200 oC for 10 hours (R-MnO2) prior to intercalation. Chemical lithiation was carried out by reacting the host compounds with LiI in acetonitrile at ~ 60 - 70 oC under a dry N2 atmosphere for 3 days (β-MnO2) or for 5 hours (RMnO2) [4]. The products were filtered and washed several times with acetonitrile and finally dried at 60 oC in air for 2 days. For comparison with previous work [5], chemical lithiation of GG9.11.1
pyrolusite (β-MnO2) with n-BuLi in hexane was also carried out. The lithium and manganese contents in the lithiated samples were determined by ICP experiments (for details se
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