Ion-Exchange Synthesis, Crystal Structure, and Electrochemical Properties of Li₂Ti₆O₁₃

Li 2 Ti 6 O 13 was prepared from Na 2 Ti 6 O 13 as a parent compound via sodium/lithium ion exchange in molten LiNO 3 at 380 °C. It crystallizes in the monoclinic system, space group C 2/ m, and with the lattice parameters of a = 15.3065(4) Å, b = 3.74739(8) Å, c = 9.1404(2) Å, and β = 99.379(2)°. The crystal structure of Li 2 Ti 6 O 13 was refined to the conventional values of R wp = 5.78% and R p = 4.44% with a fit indicator of GOF = R wp / R e = 1.66 by Rietveld analysis using powder neutron diffraction data. The crystal structure of Na 2 Ti 6 O 13 was also reinvestigated using the single-crystal X-ray diffraction data with the final R value of 2.70%. The basic (Ti 6 O 13 ) 2− framework in Li 2 Ti 6 O 13 was maintained nearly unchanged from that in the parent Na 2 Ti 6 O 13 . The Li occupation site in the tunnel space shifted to y = 0.5 position from the original Na site in Na 2 Ti 6 O 13, and the moving resulted in the LiO 4 planar coordination in Li 2 Ti 6 O 13 . The structural stability of Li 2 Ti 6 O 13 was confirmed by bond valence sums, the data of 7 Li-MAS NMR and high-temperature in situ XRD measurements, and the results of the present first-principles calculation by the FLAPW method. The electrochemical Li insertion/extraction experiments revealed the irreversible large Li insertion capacity of above 200 mAh g −1 at approximately 1.5 V for the first cycle and the stable reversible capacity of approximately 90−95 mAh g −1 during the following cycles. The total Li-ion conductivity in Li 2 Ti 6 O 13 was estimated to be σ total = 5.60 × 10 −6 S cm −1 at room temperature from the results of AC-impedance measurements. This value is comparable to that in the well-known good Li-ion conducting ramsdellite-type Li 2 Ti 3 O 7 .