Tin Oxides with Hollandite Structure as Anodes for Lithium Ion Batteries

Tin oxides, K 2 (M,Sn) 8 O 16 (M = Li, Mg, Fe, or Mn), possessing the hollandite crystal structure have been synthesized and characterized by a variety of techniques, and their electrochemical behavior was studied. The compounds K 2 (Li 2/3 Sn 22/3 )O 16 (K−Li), K 2 (Mn 2 Sn 6 )O 16 (K−Mn), and K 2 (Fe 2 Sn 6 )O 16 (K−Fe) are new phases. Rietveld refinement of the powder X-ray diffraction data showed that these Sn-hollandites exhibit a simple tetragonal structure. X-ray photoelectron spectroscopy on (K−Li) and (K−Mg) confirm formal valencies of the ions in the compounds. Galvanostatic cycling versus Li metal in the voltage range of 0.005−1.0 V at the current density of 60 mA/g showed the first cycle charge capacities of 602, 505, 481, and 418 (± 3) mAh/g for (K−Li), (K−Mg), (K−Fe), and (K−Mn), respectively. These values correspond to 3.7−3.0 mol of recyclable Li/mol of Sn. At the end of 50 cycles, (K−Li) and (K−Fe) performed better and retained 78 and 83% of the initial capacity. The (K−Li) also showed good rate capability. The Coulombic efficiency was >98% between 10 and 50 cycles in all cases except (K−Mn) with the average charge and discharge voltages of 0.4−0.5 and 0.25−0.3 V, respectively. Cyclic voltammograms complement the galvanostatic results. Impedance spectral data on the (K−Li) versus Li at different voltages during the 1st and 15th discharge−charge cycle have been analyzed and interpreted.