Glass-to-Crystal Transition in Li_1+xAl_xGe_2–x(PO₄)₃: Structural Aspects Studied by Solid State NMR

Abstract

The structural aspects of the glass-to-crystal transition in the technologically important ion conducting glass ceramic system Li 1+x Al x Ge 2−x (PO 4 ) 3 (0 ≤ x ≤ 0.75) have been examined by complementary multinuclear solid state nuclear magnetic single and double-resonance experiments. In the crystalline state, the materials form solid solutions in the NASICON structure, with additional nanocrystalline AlPO 4 present at x values ≥0.5. Substitution of Al in the octahedral Ge sites results in a binomial distribution of multiple phosphate species, which differ in the number P−O−Al and P−O−Ge linkages and can be differentiated by 31 P double quantum NMR studies suggest that the AlO 6 coordination polyhedra are noticeably expanded compared to the GeO 6 sites in the NASICON-type LiGe 2 (PO 4 ) 3 (LGP) structure. While the glassy state is characterized by a significantly larger degree of disorder concerning the local coordination of germanium and aluminum, dipolar solid state NMR studies clearly indicate that their medium range structure is comparable to that in NASICON, indicating the dominance of P−O−Al and P−O−Ge over P− O−P and Al−O−Ge connectivities. ■ INTRODUCTIONThe rapid growth in global energy demand in the face of dwindling global resources has been stimulating increasing efforts toward the development of high energy and high power batteries based on both lithium ion 1 and solid oxide fuel cell (SOFC) 2 technologies. Both processes utilize fast ion transport in the solid state at ambient or elevated temperatures. During the past two decades a large number of crystalline and glassy fast ion conductors have been identified and thoroughly characterized, and excellent reviews on oxide and proton conductors for fuel cell applications, 3 as well as fast ion conductors for lithium batteries, are available.4−6 The highest lithium ion conductivities in the solid state are generally encountered in crystalline compounds with highly disordered cation sublattices, termed superionic crystals. On the other hand, ion conducting glasses are often preferred in practice as they do not suffer from grain boundary effects and form more homogeneous interfaces with the anode and cathode compartments of a solid state electrochemical cell. The favorable features of both the crystalline and the glassy state can be combined in an ideal way using ion conducting glass ceramics, and numerous promising systems presenting electrical conductivities in excess of 10 −3 (Ω·cm) −1 at room temperature have been developed. 7,8 One particularly attractive system is based on the crystallization of precursor glasses in the systems Li 2 O-MO 2 -Al 2 O 3 -P 2 O 5 (M = Ge, Ti). While the glasses themselves are poorly conducting, they crystallize in the NASICON structure, which features exceptionally high ionic conductivities.9−15 Their structure is derived from the phases MTi 2 (PO 4 ) 3 (M = Li, Na) and MGe 2 (PO 4 ) 3 , which feature octahedrally coordinated Ti 4+ and Ge 4+ cations linked through phosphate tetrahedra via corner sharing ...

8.5 µm‐Thick Flexible‐Rigid Hybrid Solid–Electrolyte/Lithium Integration for Air‐Stable and Interface‐Compatible All‐Solid‐State Lithium Metal Batteries

“…Additional signals at −3.51 and −2.78 ppm can be assigned to lithium ions in LAGP and at the LAGP interface, respectively. [40,41] Second, XPS depth profilings reveal a LiF-rich gradient SEI, which is conducive to maximally uniformized lithium electroplating (Figure 3b). With the etching sequence, the peak of LiF (683.6 eV) in high-resolution F1s spectra gradually increases, while organic contents show a reverse trend to F1s, which represents CC/CH (284.5 eV), CO (285.7 eV), CO (286.8 eV), and CO 3 (289.3 eV) in highresolution C1s spectra (Figure S9, Supporting Information).…”

Structure and lithium‐ion mobility in Li1.5M0.5Ge1.5(PO4)3 (M = Ga, Sc, Y) NASICON glass‐ceramics

“…Figure presents the 31 P MAS NMR spectra of samples LYGP and LYGP‐C‐728. While the precursor glass only produces a broad signal, the spectrum of sample LYGP‐C‐728 consists of three prominent resonances: two overlapping signals at −45.2 and −44.1 ppm may be assigned to P 4 4Ge,0Y phosphate units whereas the one at −12.8 ppm belongs to YPO 4 . There is no indication of Ge 4+ ‐> Y 3+ substitution.…”

“…From the X‐ray powder patterns, one can see that substitution of Ge 4+ by Ga 3+ and Sc 3+ was successful, although in the latter case a multiphase crystallization pattern was observed in the DSC analysis for LSGP precursor glass and some extra peaks were also observed in the diffraction pattern. As previously shown, the exact degree of Ge substitution x in the NASICON phase can be obtained from the 31 P MAS‐NMR spectra using the expressionwhere I n refers to the fractional area of each P 4 (4‐n)Ge,nM signal component. Using the data shown in Table , values of x = 0.53 and 0.38 are calculated for LGGP‐C‐606 and LSGP‐C‐690, respectively.…”

“…In the last years, a large number of studies have focused on the Li 1+ x Al x Ge 2‐ x (PO 4 ) 3 system characterizing structural aspects and lithium‐ion mobility as well as technological applications . Further compositional optimization has been attempted by combining isovalent (Sn 4+ and Ti 4+ ) and aliovalent substitution (Al 3+ and Cr 3+ ) approaches in an effort to decrease the activation energy of M1 → M2 hopping.…”

Structure of crystalline and amorphous materials in the NASICON system Na1+xAlxGe2−x(PO4)3

“…However, it was discovered that they can also be prepared via the glass-ceramic route, giving the significant advan-air batteries and related systems [3,10]. Comparatively little is known, however, about the Na-containing materials, although solid-state nuclear magnetic resonance (NMR) experiments on the LAGP [11] and NAGP [12] systems indicate that all structural aspects of the glassto-crystal transition are the same, suggesting the same crystallization mechanisms. Homogeneous nucleation in NAGP glasses is supported by the results obtained from differential scanning calorimetry experiments on monolithic versus powdered samples [8].…”