Two-Dimensional Substitution: Toward a Better Understanding of the Structure–Transport Correlations in the Li-Superionic Thio-LISICONs

Abstract

A deeper understanding of the relationships among composition–structure–transport properties in inorganic solid ionic conductors is of paramount importance to develop highly conductive phases for future employment in solid-state Li-ion battery applications. To shed light on the mechanisms that regulate these relationships, in this work, we perform a “two-dimensional” substitution series in the thio-LISICON family Li4Ge1–x Sn x S4–y Se y . The structural modifications brought up by the elemental substitutions were investigated via Rietveld refinements against high-resolution neutron diffraction data that allowed a precise characterization of the anionic framework and lithium substructure. The analyses show that the anionic and cationic substitutions influence the polyhedral and unit cell volumes in different fashions and that the size of the polyanionic groups alone is not enough to describe lattice expansion in these materials. Moreover, we show that the lithium disorder that is crucial to achieve fast ionic mobility may be correlated to the lithium polyhedral volumes. The correlation of these structural modifications with the transport properties, investigated via electrochemical impedance spectroscopy and 7Li nuclear magnetic resonance spin-lattice relaxation measurements, shows a nonmonotonic behavior of the ionic conductivity and activation energy against the lithium polyhedral volumes, hinting to an optimal size of the conduction pathways for the ionic diffusion. Ultimately, the results obtained in this work will help to establish new guidelines for the optimization of solid electrolytes and gain a more profound understanding of the influence of the substituents on the structure and transport properties of Li-ion conductors.

Different Chemical Environments of [Ge4Se10]4– in the Li+ Compounds [Li4(H2O)16][Ge4Se10]·4.33H2O, [{Li4(thf)12}Ge4Se10], and [Li2(H2O)8][MnGe4Se10], and Ionic Conductivity of Underlying “Li4Ge4Se10”

“…Between −70 and 120 °C, the data of Li 4 Ge 4 Se 10 follow an Arrhenius behavior with an activation energy E A of 0.49 eV and an ionic conductivity at room temperature (RT) (σ RT ) of 1.7 × 10 –6 S/cm (Figure ). This value is close to the one reported for Li 4 GeSe 4 (2 × 10 –6 S/cm); , hence, it is 1 order of magnitude higher than that of Li 4 GeS 4 , the thio-LISICON parent compound, and lower than the values reported for Li 4 SnS 4 or Li 4 SnSe 4 (7 × 10 –5 S/cm, 2 × 10 –5 S/cm), , which equally situates the performance of Li 4 Ge 4 Se 10 between that of the chalcogenido tetrelates based on either [GeS 4 ] 4– or [SnE 4 ] 4– anions.…”

“…In contrast to the elemental combinations mentioned above, lithium selenido germanates have not been investigated to a great extent, with only two known compounds so far, Li 3 Ge 3 Se 6 and Li 4 GeSe 4 . , We thus extended our investigations of selenido germanates toward Li + salts in the absence or presence of solvents. Herein, we report our first resultswith regard to chemical treatment and crystallization under different conditions and with regard to ionic conductivity.…”

Exploring Aliovalent Substitutions in the Lithium Halide Superionic Conductor Li3–xIn1–xZrxCl6 (0 ≤ x ≤ 0.5)

“…1 The application of solid electrolytes improve the battery not only in terms of reduced risk of combustion, compared to liquid electrolytes, but also by enabling battery use at a wider temperature range and higher current densities for operation. 1,2 The search for the replacement of liquid electrolytes led to the discovery of several promising solid electrolytes including garnets, [3][4][5] Li10GeP2S12 6-8 , argyrodites [9][10][11] and thio-LISICONs [12][13][14][15] , all of which were investigated thoroughly by iso-or aliovalent substitutions to determine influences on ionic transport, for instance the width of diffusion pathway, 5,16,17 lattice polarizability 9,18,19 or Li + /vacancy density. 13,[20][21][22] Recently, the ternary halide electrolytes Li3MX6 (M = Y, Er, In, Sc and X = Cl, Br, I) have drawn attention for exhibiting conductivities in the mS•cm -1 -range at room temperature in combination with an enhanced electrochemical stability compared to the sulfide electrolytes.…”

Paradigms of frustration in superionic solid electrolytes

“…Diffusion may also be limited to one-dimensional channels, which can be easily blocked. The importance of having a percolating network of low-barrier transitions has [84,128,129] structural: destabilized energy minima in amorphous phase due to lack of crystalline order [95]; structural arrangement and local coordination symmetry of anions impacts diffusivity [130,131]; vacancy-induced site disorder and partial site occupancy induce mobility [91]; atomic substitution can tune lattice volume and relative cation site preference to maximize frustration [132,133]; ion conduction correlates with lack of cation site preference [134] dynamical: enhanced [PS 4 ] chemical: inductive effect through S interaction affects barriers [38][39][40]; substitution with O changes bonding [41] structural: site occupancies disorder above transition temperature [93]; Ge/P site disorder can increase conductivity [135]; O substitution for S changes site occupancy [41]; distorted intrinsic site symmetry, fluctuations in the coordination environment, and lack of clear site preference enhance frustration in Ti-based variant [58]; conduction pathway changes with local site volume [40] dynamical: enhanced [PS 4 ] 3− reorientations from structural or chemical modification lead to faster conductivity [89,93,94]; highly correlated migration in sulfide and oxygen-substituted variants [10,41] . [68] and by Morgan in the context of argyrodites [61].…”