Johannes Hartel1,Wolfgang Zeier1,2
Department of Inorganic and Analytical Chemistry1,Helmholtz-Institut2
Johannes Hartel1,Wolfgang Zeier1,2
Department of Inorganic and Analytical Chemistry1,Helmholtz-Institut2
Solid electrolytes providing high ionic conductivities are required for the realization of solid-state batteries with competitive power and energy densities. Among all reported fast lithium-ion conductors, superionic lithium argyrodites provide a promising, highly tunable compositional space, in which superior lithium-ion conductivities can be realized by elemental substitutions.<br/><br/>In this work, selenophosphate-based lithium argyrodites Li<sub>6-<i>x</i></sub>PSe<sub>5-<i>x</i></sub>Br<sub>1+<i>x</i></sub> (0 ≤ <i>x</i> ≤ 0.2) with exceptional ionic conductivities up to 8.5 mScm<sup>-1</sup> are reported and underlying reasons for their fast lithium-ion transport are unveiled based on structural characterization using a combination of neutron powder diffraction, <sup>7</sup>Li and <sup>31</sup>P NMR and Raman spectroscopy. Rietveld refinement of the neutron powder diffraction data uncovers a significantly improved interconnection of the lithium-ion cages when compared to the thiophosphate analogue Li<sub>6</sub>PS<sub>5</sub>Br as a result of the occupation of two additional lithium-ion sites, leading to enhanced lithium-ion transport. A larger unit cell volume, lattice softening and higher structural disorder between halide and chalcogenide provide further structural reasons for improved lithium-ion conduction. Interestingly, when testing Li<sub>5.85</sub>PSe<sub>4.85</sub>Br<sub>1.15</sub> as catholyte in In/LiIn|Li<sub>6</sub>PS<sub>5</sub>Br|LiNi<sub>0.83</sub>Co<sub>0.11</sub>Mn<sub>0.06</sub>O<sub>2</sub>:Li<sub>5.85</sub>PSe<sub>4.85</sub>Br<sub>1.15</sub> solid-state batteries, severe degradation is observed by electrochemical impedance spectroscopy upon charging of the cells, revealing that selenophosphate-based lithium argyrodites are not suitable for application in lithium nickel cobalt manganese oxide (NCM) based solid-state batteries from a performance perspective.<br/><br/>Besides providing further insights into the structure-transport relationship of lithium argyrodites with high lithium-ion conductivities, this work reemphasizes the necessity to consider chemical and electrochemical stability of solid electrolytes against the active materials when developing fast lithium-ion conductors.<br/><br/><b>References</b><br/>J. Hartel, A. Banik, J. M. Geredes, B. Wankmiller, B. Helm, C. Li, M. Kraft, M. R. Hansen, W. G. Zeier “Understanding lithium-ion transport in selenophosphate-based lithium argyrodites and their limitations in solid-state batteries” <i>Chem. Mater.</i><b> 2023</b>, accepted.