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 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.
Highlights
The development of advanced energy-storage technologies plays a key role in realizing electric vehicles.[1,2,3,4] Nextgeneration high-energy-density storage systems require low flammability, good electrochemical stability, and fast charging times
We report on the novel superionic conductor Li9AlP4 which is synthesised from the elements via ball-milling and subsequent annealing at moderate temperatures and which is characterized by singlecrystal and powder X-ray diffraction
Different crystalline materials have been proven to act as lithium conductors such as perovskite-type structures,[19,20,21,22] lithium superionic conductor (LISICON)-type structures,[23,24,25,26] thio-LISICON-type structures and thiophosphates,[27,28,29,30,31,32] sodium superionic conductor (NASICON)-type structures,[33,34] garnet-type structures,[35,36,37] lithium argyrodites,[38] lithium borohydrides,[39] lithium nitrides,[40,41,42] lithium hydrides,[43] and lithium halides.[44]
Summary
The development of advanced energy-storage technologies plays a key role in realizing electric vehicles.[1,2,3,4] Nextgeneration high-energy-density storage systems require low flammability, good electrochemical stability, and fast charging times. The coincidence of a higher charge carrier density due to more Li ions for charge compensation together with a large number of vacancies is considered as an important prerequisite for a higher lithium ion conductivity. This aspect must be in line with a low activation energy for lithium mobility as it occurs in structures with an effective polyhedral connectivity.[50] An aliovalent substitution of formal “Si4+” by “Al3+” and formation of [AlP4]9À instead of [SiP4]8À tetrahedra allow for the presence of an even higher number of lithium ions and a strong influence on the Li occupation in voids. Only recently, ab initio simulations suggested that doping of the moderate lithium-ion conductor Li2SiP2 with Al could enhance the ionic conductivity.[47, 54]
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