Fast Lithium Ion Conductivity in the Solid Solution Li8+X
Al
X
Ge1-X
P4 (0≤X≤1) By Aliovalent Substitution
Authors: Mueller David , Katzenmeier Leon , Restle Tassilo Manuel Florian , Fässler Thomas Friedrich 
Published: December 2020

All solid-state batteries are considered to extend the limits of lithium ion batteries in terms of power density, cycle life stability and device safety. One key component for this technology is the solid electrolyte. Heavy research on oxide and sulfide based lithium ion conductors has brought out some highly optimized systems, which show ionic conductivities up to 25 mScm-1.[1]
Recently lithium phosphides have been introduced as a new class of lithium ion conductors with ionic conductivities of 1.1 mScm-1 for Li14SiP6
[2]
and 3 mScm-1 for Li9AlP4.
[3] Due to the increased negative charge of the phosphide anion compared to conventionally used chalcogenide anions, the lithium content and therefore the charge carrier concentration is significantly higher. Since aliovalent doping can lead in sulfides to an ionic conductivity an order of magnitude higher than the parent undoped compounds, we aim here for the stepwise aliovalent substitution of the four-valent tetrel-ion in lithium phosphido-tetrelates with three-valent aluminum, to further increase the lithium concentration as well as the volume of the unit cell.[5][6] In this work we show the effect of the substitution of germanium by aluminum on the ionic conductivity in the solid solution Li8+x
Al
x
Ge1-x
P4 (0≤x≤1). β-Li8GeP4,[4] Li9AlP4 and the solid solution Li8+x
Al
x
Ge1-x
P4 crystallize in the cubic space group P-43n with the phosphorus atoms building a distorted fcc-lattice. The solid solution strictly follows Vegard’s law showing a linear dependency of the lattice parameter on the substitution grade over the whole investigated range (figure 1). Ge and Al atoms occupy 1/8 of the tetrahedral voids. Remaining tetrahedral voids and some octahedral voids are partially occupied by lithium. The cubic structure makes the system a convenient model for investigations of substitution effects, as symmetry restrictions prohibit extent distortion. Lithium ions are mobile and migrate between face sharing octahedral and tetrahedral positions as well as edge sharing tetrahedral voids. Impedance spectroscopy measurements were conducted in a custom-made cell setup for powder samples, where powders are pelletized in situ and contacted by steel electrodes. High pressure is applied over six screws, tightened with defined torque. The temperature was controlled by a specially designed heating block connected to a heating circulator, which allows to do all measurements in argon atmosphere. The ionic conductivity in Li8+x
Al
x
Ge1-x
P4 increases with the amount x of aluminum by nearly one order of magnitude. This increase does not proceed linearly, revealing a more complex correlation (figure 1).

Figure 1: Left: Vegard plot for the solid solution Li8+x
Al
x
Ge1-x
P4 (0≤x≤1) created on powder X-ray diffraction data. The black line acts as a guide to the eye, emphasizing the linear dependency of the lattice parameter a on the grade of substitution x. The upper left corner shows the crystal structure of Li8+x
Al
x
Ge1-x
P4. Right: Temperature dependency of the ionic conductivity for different compositions of Li8+x
Al
x
Ge1-x
P4.

References

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Figure 1