Günther J. Redhammer

Structural and Electrochemical Consequences of Al and Ga Cosubstitution in Li7La3Zr2O12 Solid Electrolytes

Several “Beyond Li-Ion Battery” concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to “non-garnet” (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10–4 S cm–1 to 1.2 × 10–3 S cm–1, which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm2, the lowest reported value for LLZO so far. These results illustrate that understanding the structure–properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

Structural variation and crystal chemistry of LiMe3+Si2O6 clinopyroxenes Me3+ = Al, Ga, Cr, V, Fe, Sc and In

Abstract A total of 32 synthetic end-member and solid-solution compounds of the LiM3+Si2O6 (Li = M2 site, M3+ = M1 site = Al, Ga, V, Fe, Sc and In) clinopyroxene series have been investigated by single-crystal X-ray diffraction. Except LiCrSi2O6, all compounds show C2/c symmetry at 295 K. LiCrSi2O6 has space group P21/c but transforms to the high temperature C2/c structure at 335 K. The variations of structural parameters in the LiMe3+Si2O6 clinopyroxenes are dominated by the Me3+ site. The average M1—O bond length is linearly correlated with the ionic radius of the M1 cation. Octahedra reflect the increasing size of the M1 cations by steadily increasing bond and edge lengths, the variations however are not uniform. With increasing size of the M1 cation, octahedra deviate from ideal octahedral geometry. Octahedral edges, shared with other structural units, are distinctly hampered in expansion with increasing size of the M1 cation. The increasing size of the M1 octahedral chain is compensated by changing the kinking of the tetrahedral chain and by alterations of bond and edge lengths as well as the bond angles within the tetrahedron. Three different mechanisms of adapting of the structural building units with increasing M3+ cationic radius can be identified: (i) expansion of the tetrahedral chain by stretching (ii) transition form “O” to “S” chain conformation after full expansion and (iii) finally a limit of expansion in a direction. We stress that cations larger than In3+ cannot be substituted at the M1 site because of too large geometrical differences between octahedral and tetrahedral chains.

Structural changes upon the temperature dependent C2/c → P21/c phase transition in LiMe3+Si2O6 clinopyroxenes, Me = Cr, Ga, fe, V, Sc and In

Abstract Within the Li-clinopyroxene series LiMe3+Si2O6, a temperature induced C2/c → P21/c phase transition has been observed for Me = Cr, Ga, V, and Sc at temperatures of 330(1), 286(1), 205(3) and 234(1) K, respectively. There is no phase transition for the Al3+ and the pure In3+ compound down to 80 K. Within the LiSc1–xInxSi2O6 solid solution se ries, the transition temperature rapidly decreases with increasing In3+ content and drops below 90 K between x = 0.26 and 0.30. The C → P phase transition is found only in samples with nearly fully extended tetrahedral chains. The phase transition in LiScSi2O6, with its “O-rotated” and distinctly more kinked tetrahedral chains in C2/c (O3—O3—O3 angle = 175.7(1)° at 298 K), exhibits a different character: the decay of the b-type Bragg reflections hkl: h + k ≠ 2n and the structural changes in the vicinity of the phase transition are less rapid. The temperature dependent evolution of the order parameter Q2 (as expressed by the decay of the b-type Bragg reflections) suggests a second order thermodynamic character of the C → P phase transition in the Sc3+ compound, whereas it is close to a tri-critical behaviour in the other ones. The structural changes, taking place at the phase transition and below are similar to those in LiFeSi2O6, i.e. dislinkage and appearance of two differently kinked tetrahedral chains in P21/c, decrease of the coordination number of Li+ from 6 to 5, slight alterations of octahedral bond lengths. The structural changes at the phase transition are of similar magnitude in the Ga, Cr, V and Fe compound. LiScSi2O6 is again exceptional. Different thermal expansion of octahedral and tetrahedral sites and increasing site distortion of the M1 site as a consequence of tetrahedral chain kinking in C2/c are assumed to be factors inducing the C2/c → P21/c phase transition.

Single-crystal structure refinement of NaTiSi2O6 clinopyroxene at low temperatures (298 < T < 100 K).

The alkali-metal clinopyroxene NaTi(3+)Si2O6, one of the rare compounds with trivalent titanium, was synthesized at high temperature/high pressure and subsequently investigated by single-crystal X-ray diffraction methods between 298 and 100 K. One main difference between the high- and the low-temperature form is the sudden appearance of two different Ti(3+)-Ti3+ interatomic distances within the infinite chain of the TiO6 octahedra just below 197 K. This change can be seen as direct evidence for the formation of Ti-Ti singlet pairs in the low-temperature phase. Mean Ti-O bond lengths smoothly decrease with decreasing temperature and the phase transition is associated with a slight jump in the Ti-O bond length. The break in symmetry, however, causes distinct variations, especially with respect to the two Ti-O(apex) bond lengths, but also with respect to the four Ti-O bonds in the equatorial plane of the octahedron. The TiO6 octahedron appears to be stretched in the chain direction with a slightly larger elongation in the P1; low-temperature phase compared with the C2/c high-temperature phase. Polyhedral distortion parameters such as bond-length distortion and octahedral angle variance suggest the TiO6 octahedron in P1; to be closer to the geometry of an ideal octahedron than in C2/c. Mean Na-O bond lengths decrease with decreasing temperature and the variations in individual Na-O bond lengths are the result of variations in the geometry of the octahedral site. The tetrahedral site acts as a rigid unit, which does not show pronounced changes upon cooling and through the phase transitions. There are neither large changes in bond lengths and angles nor in polyhedral distortion parameters, for the tetrahedral site, when they are plotted. In contrast with the C2/c-->P2(1)/c phase transition, found especially in LiMSi2O6 clinopyroxenes, no very large variations are found for the tetrahedral bridging angle. Thus, it is concluded that the main factor inducing the phase transition and controlling the structural variations is the M1 octahedral site.

A microcontact impedance study on NASICON-type Li1+xAlxTi2−x(PO4)3 (0 ≤ x ≤ 0.5) single crystals

We successfully demonstrated the applicability of microcontact impedance spectroscopy (MC IS) on Li+ conducting solid electrolytes and measured the Li+ bulk conductivity (σb) of LiTi2(PO4)3 (LTP) and Li1+xAlxTi2−x(PO4)3 (LATP) single crystals independent of microstructural effects (e.g., grain boundaries, pores, and density). The crystals had a size of about 100 μm in each direction and crystallized with NASICON-type structure (Rc). Finite element calculations were performed to validate the impedance data analysis. A strong increase in σb in the order of three magnitudes (3.16 × 10−6 to 1.73 × 10−3 S cm−1) was found after incorporating 0.1 mol Al3+ per formula unit into LTP. Moreover, since the crystal structural changes are almost linear in the LATP system up to x = 0.5, the increase of σb is most probably related to additional Li+ sites at the M3 (36f) position. The additional Li+ leads to a displacement of Li+ occupying the M1 (6b) sites towards the nearest-neighboring M3 position, and therefore opens the fast-conducting pathway within the NASICON structure. A significant change in σb was also observed as the Al3+ content further increased (x = 0.1 to 0.5). The highest σb value of 5.63 × 10−3 S cm−1 was obtained for samples with x = 0.4.

Fast Li-Ion-Conducting Garnet-Related Li7–3xFexLa3Zr2O12 with Uncommon I4̅3d Structure

Fast Li-ion-conducting Li oxide garnets receive a great deal of attention as they are suitable candidates for solid-state Li electrolytes. It was recently shown that Ga-stabilized Li7La3Zr2O12 crystallizes in the acentric cubic space group I4̅3d. This structure can be derived by a symmetry reduction of the garnet-type Ia3̅d structure, which is the most commonly found space group of Li oxide garnets and garnets in general. In this study, single-crystal X-ray diffraction confirms the presence of space group I4̅3d also for Li7–3xFexLa3Zr2O12. The crystal structure was characterized by X-ray powder diffraction, single-crystal X-ray diffraction, neutron powder diffraction, and Mößbauer spectroscopy. The crystal–chemical behavior of Fe3+ in Li7La3Zr2O12 is very similar to that of Ga3+. The symmetry reduction seems to be initiated by the ordering of Fe3+ onto the tetrahedral Li1 (12a) site of space group I4̅3d. Electrochemical impedance spectroscopy measurements showed a Li-ion bulk conductivity of up to 1.38 × 10–3 S cm–1 at room temperature, which is among the highest values reported for this group of materials.

Synthetic LiAlGe2O6: The first pyroxene with P21/n symmetry

Abstract The structure determination of synthetic LiAlGe2O6 [Z = 4, space group P21/n, a = 9.8892(5), b = 8.3929(5), c = 5.3995(3) Å, b = 110.646(3)°], is a new pyroxene structure-type, and represents the first structural example of P21/n pyroxene symmetry. The crystal structure of the Ge-analog phase of spodumene was solved from single-crystal X-ray diffraction data by classical Patterson methods with a subsequent structure refinement converging to R1 = 0.0169. The new P21/n pyroxene polymorph was found to consist of a single S-type rotated tetrahedral chain type, which is-similar to ordered P21/n omphacite-composed of alternating Ge1O4 and Ge2O4 tetrahedra located at two distinct sites within a single chain. This (Ge1Ge2O6)n chain is S-rotated, strongly bent (O4-O2-O4 = 154.8°) compared to that of the C2/c spodumene structure, and assumes angles comparable to those of the two chains of spodumene at 3.3 GPa within the P21/c symmetry. As a consequence of the interplay between the M1 and M2 sites, the new polymorph reveals a larger angular distortion for the AlO6 octahedra, and the Li coordination is reduced from sixfold to fivefold coordination. This establishes Li[5]Al[6](Ge1[4] Ge2[4]O6) as the corresponding crystallochemical formula for the new P21/n representative within the monoclinic (clino)pyroxene family.

Crystal and magnetic spin structure of Germanium-Hedenbergite, CaFeGe2O6, and a comparison with other magnetic/magnetoelectric/multiferroic pyroxenes

Abstract CaFeGe2O6, the germanium-analogue to the mineral Hedenbergite, has been synthesized at 1273 K in evacuated SiO2-glass-tubes. Powder neutron diffraction data collected between 4 K and 300 K were used to evaluate the magnetic spin as well as the nuclear crystal structure and its T-evolution. CaFeGe2O6 is monoclinic, C2/c, a = 10.1778(5) Å, b = 9.0545(4) Å, c = 5.4319(3) Å, β = 104.263(3)°, Z = 4 at room temperature. No change of symmetry was observed down to 4 K. Below 43 K, additional magnetic Bragg reflections appear, which can be indexed on the basis of a commensurate magnetic propagation vector k [1, 0, 0]. The successful description of the magnetic spin structure reveals a ferromagnetic spin coupling within the Fe2+O6 M1 chains, while the coupling between the chains is antiferromagnetic. Spins are oriented collinearly within the a–c plane and form an angle of ∼60° with the crystallographic a-axis. The magnetic moment at 4 K amounts to about 4.4 μB. The observed magnetic structure is similar to that of other Ca-clinopyroxenes. The present data are put into context with the structural and magnetic properties of other pyroxenes – among them magnetoelectric and multiferroic pyroxene-type compounds.

Magnetic and low-temperature structural behavior of clinopyroxene-type FeGeO3: A neutron diffraction, magnetic susceptibility, and 57Fe Mössbauer study

Abstract The clinopyroxene-type compound FeGeO3 was synthesized using ceramic sintering techniques at 1273 K in evacuated silica tubes and investigated by powder neutron diffraction between 4 and 300 K, X-ray diffraction, SQUID magnetometry, and 57Fe Mössbauer spectroscopy. The title compound shows space group C2/c symmetry (high pressure, HP-topology) between 4 and 900 K. No structural phase transition is present within this temperature interval, whereas lattice parameters show discontinuities around 50 and 15 K, which are due to magnetic phase transitions and the associated magneto-elastic coupling of the lattice. The magnetic susceptibility data show two maxima in their temperature dependence, one at ~47 K, the second around 12 K (depending on the external field), indicative of two magnetic transitions in the title compound. Neutron data shows that for T < 45 K, FeGeO3 orders magnetically, having a simple collinear structure, with space group C2/c, and with the spins aligned parallel to the crystallographic b-axis, both on M1 and M2. The coupling within the M1/M2 band is ferromagnetic, whereas between them it is antiferromagnetic. As the bulk magnetic measurements in the paramagnetic state revealed a dominating ferromagnetic coupling, the intra-chain interactions dominate the inter-chain interaction. At 12 K, additional magnetic reflections appear, revealing a second magnetic phase transition. Spins are rotated away from the b-axis toward the a-c plane. The coupling within the M1 chain is still ferromagnetic and antiferromagnetic between the M1 chains. However, spins on M1 and M2, are no longer collinear. The moment on the M2 site is rotated further away from the b-axis than on M1.

On the crystal chemistry of olivine-type germanate compounds, Ca1 + xM1 − xGeO4 (M2+ = Ca, Mg, Co, Mn)

Germanate compounds, CaMGeO(4) with M(2+) = Ca, Mg, Co and Mn, were synthesized as single crystals by slow cooling from the melt or by flux growth techniques. All the compositions investigated exhibit Pnma symmetry at 298 K and adopt the olivine structure. The M2 site is exclusively occupied by Ca(2+), while on M1 both Ca(2+) and M(2+) cations are found. The amount of Ca(2+) on M1 increases with the size of the M1 cation, with the smallest amount in the Mg compound (0.1 atoms per formula unit) and the largest in the Mn compound (0.20 atoms per formula unit), while in Ca(2)GeO(4), also with olivine structure, both sites are completely filled with Ca(2+). When compared with those of Ca silicate olivine, the lattice parameters a and c are distinctly larger in the analogous germanate compounds, while b has essentially the same values, regardless of the tetrahedral cation, meaning that b is independent of the tetrahedral cation. Structural variations on the octahedrally coordinated M1 site are largely determined by the size of the M1 cation, the average M1-O bond lengths being identical in Ca silicate and Ca germanate olivine. Increasing the size of the M1 cation induces an increasing polyhedral distortion, expressed by the parameters bond-length distortion, octahedral angle variance and octahedral quadratic elongation. However, the Ca germanate olivine compounds generally have more regular octahedra than the analogous silicates. The octahedrally coordinated M2 site does not exhibit large variations in structural parameters as a consequence of the constant chemical composition; the same is valid for the tetrahedral site.