Publication date: 10th April 2024
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 et al. in space group C2/c with cell dimensions a = 9.716 Å, b = 11.536 Å, c = 10.697 Å and b = 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.
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.[3] 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.
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.
China Scholarship Council (Grant No. 202008440419)
neutron beam time award at ISIS (RB2410442)
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