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 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.

Synthesis, Structure, Solid‐State NMR Spectroscopy, and Electronic Structures of the Phosphidotrielates Li3AlP2 and Li3GaP2

“…As expected, the Al−P bonds are slightly shorter than the Ga−P bonds. The Al−P bond lengths are very similar to those in LiNa 2 AlP 2 (2.410(3)–2.426(3) Å) and are in the range of other known compounds with strong Al−P interactions like in AlP (2.360 Å), Na 3 AlP 2 (2.376(4) Å) or in Sr 3 Al 2 P 4 (2.377(3)–2.417(2) Å) and weaker Al−P interactions like in Li 9 AlP 4 (2.423(2)–2.434(1) Å) . In the case of Tr =Ga, the Ga−P bonds are longer than in GaP (2.3601(1) Å) and slightly shorter than in Ba 3 GaP 3 (2.43(1) Å), where [Ga 2 P 6 ] 6− dimers occur .…”

“…Impedance spectroscopy : The electrochemical impedance spectroscopy for Li 3 Tr P 2 was performed in an in‐house designed cell. The detailed setup and procedure are described in Restle et al . Impedance spectra were recorded on a Bio‐Logic potentiostat (SP‐300) in a frequency range from 7 MHz to 50 mHz at a potentiostatic excitation of ±50 mV.…”

“…Compared to the related sulfide‐based lithium ion conductors, the anionic substructure of phosphido‐based conductors carry one additional charge (formal “P 3− ” versus a formal “S 2− ”), and thus the Li content that is required for charge balance is higher. Recently, we expanded this concept of highly charged tetrahedra to lithium phosphidoaluminates by replacing the central group 14 metal by aluminium …”

Supertetrahedral polyanionic network in the first lithium phosphidoindate Li3InP2 – structural similarity to Li2SiP2 and Li2GeP2 and dissimilarity to Li3AlP2 and Li3GaP2

“…The 6 Li NMR spectrum shows only one signal with a chemical shift of 7 Li MAS-NMR spectroscopy), and Li 2 GeP 2 (3.6 and 2.4 ppm). 8,13,15,16 Compared to the abovementioned compounds with two signals in the 6 Li NMR spectrum, the difference in local coordination, which is expressed by the P-Li-P angles, is the lowest for -59.9 , -164.8 and -178.4 ppm) due to the deshielding of the more electronegative tetrel elements compared to indium. 8,13 Chemical Science Accepted Manuscript In order to gain additional insight into the experimentally observed structure types, we performed DFT-based structural optimizations for the Al, Ga and In compounds using the PBEsol functional 24 as implemented in CASTEP 25 (computational details are given in the ESI).…”

“…In recent investigations we expanded this class of compound further to phosphidoaluminates, which contain tetrahedral AlP 4 building units, and we discovered the fast lithium ion conductor Li 9 AlP 4 , which shows ionic conductivities of 3 × 10 −3 S•cm −1 . 15 In addition, we also obtained Li 3 AlP 2 , which is built up by 2 ∞ [AlP 2 3 -] layers of corner-and edge-sharing AlP 4 tetrahedra, 16 and we then also introduced the isotypic gallium compound Li 3 GaP 2 as the first phosphidogallate. 16 Both trielate (Tr = Al, Ga) compounds do not show moderate lithium ion conductivity but unexpectedly turned out to be direct band gap semiconductors with optical band gaps of 3.1 and 2.8 eV, respectively.…”