Extremely High Li+ Diffusivity in NASICON-type Li1.5Al0.5Ti1.5(PO4)3
V. Epp
Christian Doppler Laboratory for Lithium Batteries, Institute of Chemistry and Technology of Materials, Graz University of Technology
15:00 - 17:00 Monday 28 September 2015 Foyer Alte Technik The continuing demand for efficient electrochemical energy storage systems is hoped, in the medium term, to be met by rechargeable Li-based batteries. This requires improved battery characteristics in terms of power, capacity, longevity, and safety compared with the state-of-the-art Li-ion technology. Especially the last point can be addressed by the use of so-called all-solid-state batteries which do not rely on highly flammable solvent-based electrolytes but instead on solid compounds such as Li-bearing sulfides or oxides. So far, only few examples have been found whose ionic conductivities are comparable to those of liquid electrolytes.
Here, we present results on one of such promising materials viz. Li1.5Al0.5Ti1.5(PO4)3 (LATP) being a NASICON-type fast ion conductor.[1] LATP was prepared by a novel sol-gel method;[2] bulk ion dynamics were studied via various time-domain 7Li NMR techniques. The latter allowed for an in-depth investigation of Li-ion translational dynamics on different time and length scales. As an example, 7Li NMR line-shape analysis reveals that the onset of significant Li+ exchange processes, which are characterized by jump rates in the order of several kHz, is well below 140 K. Line shapes provide a first qualitative indication for a high ionic diffusivity in LATP. The finding is corroborated by 7Li NMR spin–lattice relaxation (SLR) measurements carried out in both the laboratory and the rotating frame of reference. The diffusion-induced SLR NMR rate peaks recorded with the spin-lock technique point to a mean Li+ jump rate in the order of 4×105 1/s at ca. 200 K; the corresponding activation energy is as low as 0.16 eV, thus, guaranteeing fast dynamics over a broad temperature range. Using the Einstein-Smoluchowski relation,[3,4] the jump rate determined can be translated into a bulk Li+ self-diffusion coefficient of about 10−14 m2/s. For comparison, for the majority of fast ion conductors such values are usually reached at temperatures being equal or significantly higher than 300 K. The appearance of an SLR NMR rate peak well below room temperature marks LATP as an extremely fast ion conductor. This is corroborated by additional impedance measurements showing high Li-ion conductivities of 5.3×10−4 and 1.04×10−3 S/cm in dry argon and ambient air, respectively. The difference observed points to a small influence of humidity on the grain boundary conductivity. In summary, our results reveal ultrafast Li+ dynamics in LATP being comparable to that recently found in other potential solid electrolytes such as garnets[5,6] and argyrodites.[7,8]
References:
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[2] Q. Ma, F. Tietz, O. Guillon, German Patent DE 10 2014 012 926 (2015).
[3] A. Einstein, Ann. Phys. 322, 549 (1905).
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[6] H. Buschmann, J. Dölle, S. Berendts, A. Kuhn, P. Bottke, M. Wilkening, P. Heitjans, A. Senyshyn, H. Ehrenberg, A. Lottnyk, V. Duppel, L. Kienle, J. Janek, Phys. Chem. Chem. Phys. 13, 19378 (2011).
[7] V. Epp, Ö. Gün, H.-J. Deiseroth, and M. Wilkening, J. Phys. Chem. Lett. 4, 2118 (2013).
[8] V. Epp, Ö. Gün, H.-J. Deiseroth, and M. Wilkening, Phys. Chem. Chem. Phys. 15, 7123 (2013).
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Institute of Solid State Physics
