Fast Lithium Ion Conduction in Lithium Phosphidoaluminates

Abstract

Solid electrolyte materials are crucial for the development of high‐energy‐density all‐solid‐state batteries (ASSB) using a nonflammable electrolyte. In order to retain a low lithium‐ion transfer resistance, fast lithium ion conducting solid electrolytes are required. We report on the novel superionic conductor Li 9 AlP 4 which is easily synthesised from the elements via ball‐milling and subsequent annealing at moderate temperatures and which is characterized by single‐crystal and powder X‐ray diffraction. This representative of the novel compound class of lithium phosphidoaluminates has, as an undoped material, a remarkable fast ionic conductivity of 3 mS cm −1 and a low activation energy of 29 kJ mol −1 as determined by impedance spectroscopy. Temperature‐dependent 7 Li NMR spectroscopy supports the fast lithium motion. In addition, Li 9 AlP 4 combines a very high lithium content with a very low theoretical density of 1.703 g cm −3 . The distribution of the Li atoms over the diverse crystallographic positions between the [AlP 4 ] 9− tetrahedra is analyzed by means of DFT calculations.