Li-Ion Distribution and Local Structure in the Li-Ion Conductor LiTa₂PO₈

In recent years there has been an increasing interest in lithium ion conducting solid electrolytes for use in lithium batteries, due to safety concerns with the flammable liquid electrolyte system. A new phosphate LiTa₂PO₈ (LTPO) has been found with Li-ion conductivity in the order of 10^-4 S cm^-1 at room temperature.^[1] The crystal structure of LTPO has been identified by Kim <i>et al.</i> in space group <i>C</i>2/<i>c</i> with cell dimensions <i>a</i> = 9.716 Å, <i>b</i> = 11.536 Å, <i>c</i> = 10.697 Å and <i>b</i> = 90.04°. The structure contains a framework of corner sharing TaO₆ and PO₄ polyhedra, generating various voids with Li⁺ ions distributed over three crystallographically distinct sites.^[1] Based on results from a recent neutron diffraction experiment, we have now identified between four and five sites for Li⁺ ions in the structure depending on the synthesis conditions. Using a 2-step method for the synthesis of LTPO, we have managed to reduce the grain boundary resistance, yielding a total conductivity of 1 × 10^-3 S cm^-1 at room temperature, with an activation energy of 0.35 eV.<br/>Interestingly, although the crystallographic analysis indicates the presence of a single crystallographic position for phosphorous in the average structure, the ³¹P MAS-NMR data indicate multiple resonances, suggesting a number of different local structure motifs.^[2] 2D-NMR and neutron total scattering experiments were conducted to study the local structure. ³¹P-⁶Li dipolar heteronuclear multiple-quantum coherence (D-HMQC) experiments reveal a broad ⁶Li peak made up of at least two Li species that couple with two ³¹P species, one is coupled to both P species while other Li species is coupled to only one phosphorous. This appears to correlate with our neutron diffraction results which reveal Li⁺ ions are disordered over 4 crystallographic sites; one of these sites has relatively short contacts to two neighbouring phosphate tetrahedra, while the other 3 sites neighbour only a single phosphate tetrahedron.<br/>Our analysis of total neutron scattering data on the LTPO system has shown the possibility of accurately establishing the Li⁺ ion distribution and local structure. Details of these are presented and the difference between samples prepared using different synthesis routes is discussed in light of the local structure.<br/><br/>References<br/>1. J. Kim, J. Kim, M. Avdeev, H. Yun, S. J. Kim, <i>J. Mater. Chem. A</i>, 6 (2018) 22478–22482.<br/>2. K. Kwatek, W. Slubowska-Walkusz, E. Kwiatkowska, J. L. Nowinski, A. T. Krawczynska, I. Sobrados, V. Diez-Gómez, J. Sanz, <i>J. Eur. Ceram. Soc</i>., 43 (2023) 5548–5556.