Anomalies in Bulk Ion Transport in the Solid Solutions of Li₇La₃M₂O₁₂ (M = Hf, Sn) and Li₅La₃Ta₂O₁₂

Abstract

Cubic Li 7 La 3 Zr 2 O 12 (LLZO), stabilized by supervalent cations, is one of the most promising oxide electrolyte to realize inherently safe all-solid-state batteries. It is of great interest to evaluate the strategy of supervalent stabilization in similar compounds and to describe its effect on ionic bulk conductivity σ′ bulk . Here, we synthesized solid solutions of Li 7– x La 3 M 2– x Ta x O 12 with M = Hf, Sn over the full compositional range ( x = 0, 0.25...2). It turned out that Ta contents at x of 0.25 (M = Hf, LLHTO) and 0.5 (M = Sn, LLSTO) are necessary to yield phase pure cubic Li 7– x La 3 M 2– x Ta x O 12 . The maximum in total conductivity for LLHTO (2 × 10 –4 S cm –1 ) is achieved for x = 1.0; the associated activation energy is 0.46 eV. At x = 0.5 and x = 1.0, we observe two conductivity anomalies that are qualitatively in agreement with the rule of Meyer and Neldel. For LLSTO, at x = 0.75 the conductivity σ′ bulk turned out to be 7.94 × 10 –5 S cm –1 (0.46 eV); the almost monotonic decrease of ion bulk conductivity from x = 0.75 to x = 2 in this series is in line with Meyer–Neldel’s compensation behavior showing that a decrease in E a is accompanied by a decrease of the Arrhenius prefactor. Altogether, the system might serve as an attractive alternative to Al-stabilized (or Ga-stabilized) Li 7 La 3 Zr 2 O 12 as LLHTO is also anticipated to be highly stable against Li metal.

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

“…The higher the activation enthalpy, the higher the pre-factor, which is affected by entropy contributions, attempt frequencies and, in the case of ionic conductivity, also by the effective number of charge carriers. Recently, a similar trend has also been found for LLZO-type conductors [71] and other compounds such as thiophosphates [31,72]. The semi-empirical relationship seems to be valid at least for the averaged, macroscopic ion transport.…”

Enhancing ionic conductivity in solid electrolyte by relocating diffusion ions to under-coordination sites

“…As shown in fig. S5, their alike resonances observed in the 17 O ssNMR spectra show that NZSP and LZSP crystals have a similar skeleton structure. In the skeleton structure, Wyckoff sites of Zr, Si/P, and O ions fully occupy the Wyckoff sites of 8f, 4e/8f, and 8f, respectively (figs.…”

“…Although its phase stability temperature of 742°C is lower than that (1000°C) of usual NASICON-type solid electrolytes (12,35), this temperature is much higher than the reported cathode dissociation temperature in the previous literature (36). Therefore, a hot-pressing sintering process at 700°C for 1 hour under a fixed pressure of 70 MPa was carried out to prepare LZSP pellets (the synchrotron XRD and 17 O ssNMR spectra in fig. S17 prove that the structure of LZSP annealed under 700°C keeps unchanged).…”

“…The retained skeleton structure is further supported with 17 O solidstate nuclear magnetic resonance (ssNMR), which is a high-sensitive technique to characterize the local structure of oxides at the atomic level (31,32). As shown in fig.…”

“…In the past decades, a great deal of effort has been made for developing oxide solid electrolytes such as LISICON ( 14 ), NASICON ( 15 , 16 ), perovskite ( 17 , 18 ), garnet ( 10 , 19 ), and LiPON ( 20 ) systems. The basic step in diffusion paths is Li-ion migration between two stable sites through the high-energy transition state.…”