A New Class of Superionic Solid-State Lithium-Ion Conductors: Lithium-Phosphido Silicates, Germanates, and Aluminates
Authors: Fässler Thomas Friedrich 
Published: December 2020

Superionic conductors as a solid-state electrolyte materials are essential for use for all-solid-state lithium ion batteries. Whereas intensive research on oxide and sulfide based lithium ion conductors resulted in optimized materials which reach ionic conductivities up to 25 mScm-1 other compound classes are disregarded. A design principle for fabricating new superionic conductors follows the ideas of increasing the carrier densities, changing the diffusion pathways of the mobile species, creating vacancies or increasing structural defects, and including anions that are more polarizable. For example to increase the carrier density, an effective way is especially the aliovalent substitution of cations: for example, in Li3PS4 making a formal substitution from “P5+” to “Ge4+” arises in Li3.25Ge0.25P0.75S4 with a four times higher ionic conductivity.
We propose here phosphide-based materials as ionic conductors that result in principle from the aliovalent substitution of [TtS4]4- tetrahedra, which are an important key building block in sulfide-based electrolytes, by [TtP4]8- leading to the analogous complex anions based on phosphorus (Tt = tetrel element = Group-14 element). The resulting higher charge of the [TtP4]8- tetrahedra allows for twice the number of lithium ions per formula unit for charge compensation if compared to [TtS4]4- and in addition, the more negatively charged P3- is more polarizable than S2-. In this context we discovered for example Li8SiP4,[1]

β-Li8GeP4,[2]
and Li14SiP6
[3] with conductivities up to 1.1 mScm-1
. In addition we recently succeded to synthesize the lithium richest representative, obtained by formal aliovalent substitution of “Tt
4+” by “Al3+” and formation of in Li9AlP4.[4] The insertion of Al3+ leads to formally nine-fold negatively charged [AlP4]9- tetrahedra, resulting in an even higher lithium density per formula unit and a change in spatial extent of the diffusion pathways. This first representative of a 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.
In this paper we give an overview on the recent development in this material class. All crystalline materials are readily obtained as single phase materials by ball mill synthesis from the elements followed by moderate thermal treatment of the mixtures. In all cases the atomic structures are characterized by X-ray diffraction methods. The lithium-ion mobility was investigated by temperature-dependent nuclear magnetic resonance and electrochemical impedance spectroscopy revealing low activation energies for lithium hopping. In some cases lithium diffusion pathways via both, tetrahedral and octahedral voids are analyzed by temperature-dependent powder neutron diffraction measurements in combination with maximum entropy method (MEM) and DFT calculations. The role of lithium ion occupation in the various tetrahedral and octahedral sites that occur in those compounds is highlighted.
References
[1] L. Toffoletti, H. Kirchhain, J. Landesfeind, W. Klein, L. van Wüllen, H. A. Gasteiger, T. F. Fässler, Chem. Eur. J.
22, 17635 (2016).
[2] H. Eickhoff, S. Strangmüller, W. Klein, H. Kirchhain, C. Dietrich, W. G. Zeier, L. van Wüllen, T. F. Fässler, Chem. Mater.
30, 6440 (2018).
[3] S. Strangmüller, H. Eickhoff, D. Müller, W. Klein, G. Raudaschl-Sieber, H. Kirchhain, C. Sedlmeier, V. Baran, A. Senyshyn, V. L. Deringer et al., J. Am. Chem. Soc.,
141, 14200 (2019).
[4] T. M. F. Restle, C. Sedlmeier, H. Kirchhain, W. Klein, G. Raudaschl-Sieber, V. L. Deringer, L. van Wüllen, H. A. Gasteiger, T. F. Fässler, Angew. Chem. Int. Ed., 59, 5665 (2020).
Figure 1: (left) Nyquist plot of Li9AlP4 and temperature-dependent evolution of the 7Li lineshape recorded in the temperature range from 132 to 290, (right) a 2D section cut of the lithium one-particle-potential (OPP, red ≙ low, blue ≙ high) and its 1D section along dashed lines connecting five Li atoms in a chain in Li14SiP6. Li and P are shown as grey and pink spheres, respectively.

Figure 1