Rapid Lithium Insertion and Location of Mobile Lithium in the Defect Perovskite Li_0.18Sr_0.66Ti_0.5Nb_0.5O₃

Abstract

Solids which exhibit Li-ion conductivity find applications as solid electrolytes or electrode materials in rechargeable lithium ion batteries. Since the discovery of very high ionic conductivity in Li 3x La 0.66-x TiO 3 , [1] perovskite type oxides have received increased attention due to their compositional and structural flexibility, allowing for finely tuned properties. In order to successfully design new defect perovskites with improved ionic conductivity, thorough structural investigations are necessary. While these have been performed for the Li 3x La 0.66-x TiO 3 series, [2] this is not the case for the Li 2y Sr 1-x-y Ti 1-2x Nb 2x O 3 defect perovskite type series, members of which were reported to exhibit comparable ionic conductivity. [3] The location of lithium within the structure and how it interacts with the host material is particularly important to determine diffusion pathways and the origins of any energy barriers, which must be overcome for the ions to diffuse. It is, however, often difficult to locate lithium in Li-ion conductors chiefly owing to the very low X-ray scattering factor of Li. In addition, the high Li-ion mobility and resulting disorder in these conductors might further exacerbate the problems. In contrast, neutron scattering is well suited so at least if Li-ions are ordered they can be located. The need for generally very sizeable samples is often a challenge. The current study presents a thorough structural investigation of Li 0.18 Sr 0.66 Ti 0.5 Nb 0.5 O 3 (LSTN) in conjunction with electrochemical property measurements.A synchrotron X-ray powder diffraction pattern of LSTN revealed an ideal cubic (Pm " 3m) perovskite type structure ( Figure 1). The absence of A-site ordering suggests that a three-dimensional Li diffusion pathway is likely, a hypothesis supported by bond valence sum calculations for lithium. The surfaces in Figure 2 a map the points where the bond valence sums for lithium equal 1 AE 0.1, providing an estimate for likely lithium positions. This region maps out a three-dimensional pathway, which wraps around the centre of the vacant A-site and connects adjacent vacant sites via an oxygen window in the unit cell face (Figure 2 b).Possible ordering of lithium within this region of the unit cell was determined from neutron powder diffraction data. Despite the higher resolution and intensity of synchrotron X-ray data only neutron diffraction allows for the unambiguous determination of Li positions. For X-ray diffraction the scattering factors are proportional to an atom's number of electrons, that is, Li does not scatter well compared to transition metals. For neutron diffraction the scattering length of lithium (À1.9 fm) compares favorably with both strontium (7.02 fm) and oxygen Figure 1. Synchrotron X-ray powder diffraction pattern of the as-synthesised Li 0.18 Sr 0.66 Ti 0.5 Nb 0.5 O 3 modelled with a cubic (Pm " 3m) unit cell. The upper curve corresponds to the model fitted to the data (black crosses) and the lower curve corresponds to the difference between them...

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