Fast Lithium Ion Conduction in Lithium Phosphidoaluminates

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 Li₉AlP₄ 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 ⁷Li NMR spectroscopy supports the fast lithium motion. In addition, Li₉AlP₄ 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 [AlP₄]⁹⁻ tetrahedra is analyzed by means of DFT calculations.