Abstract
Cubic Li7La3Zr2O12(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 Li7–xLa3M2–xTaxO12 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 Li7–xLa3M2–xTaxO12. 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 Ea is accompanied by a decrease of the Arrhenius prefactor. Altogether, the system might serve as an attractive alternative to Al-stabilized (or Ga-stabilized) Li7La3Zr2O12 as LLHTO is also anticipated to be highly stable against Li metal.
Highlights
To the best of our knowledge, no study so far is available in literature that systematically looked at the change of bulk ionic conductivities of the two solid solutions of Li7−xLa3Hf2−xTaxO12 (LLHTO) and Li7−xLa3Sn2−xTaxO12(LLSTO) over the full compositional range
Similar observations were reported by Gupta et al, who managed to stabilize LLHTO with a Ta amount corresponding to x = 0.2.22 We see that with increasing x the cubic phase remains the stable polymorph
The samples were characterized by Xray diffraction to identify the phases formed
Summary
Garnet-type Li7La3Zr2O12 (LLZO), if stabilized in its cubic modification by aliovalent doping, belongs to the most promising solid electrolytes[1] for all-solid-state batteries (ASSBs).[2,3] Despite of its high Li-ion conductivity in the mS cm−1 range,[4,5] LLZO has attracted great attention because of its nonflammability, high chemical and electrochemical stability, as well as its mechanical robustness.[2,6]. Apart from pure tetragonal LLZO, other tetragonal phases such as Li7La3Hf2O12 (LLHO)[17,18] and Li7La3Sn2O12 (LLSO)[19] exist which might be transformed into powerful electrolytes by substitution strategies[2] using supervalent cations. Such investigations are expected to be helpful in refining our understanding of ionic conduction in garnet-type electrolytes. To the best of our knowledge, no study so far is available in literature that systematically looked at the change of bulk ionic conductivities of the two solid solutions of Li7−xLa3Hf2−xTaxO12 (LLHTO) and Li7−xLa3Sn2−xTaxO12(LLSTO) over the full compositional range. This increase is accompanied by a prefactor anomaly which is consistent with the so-called Meyer-Neldel rule.[23,24] This compensation rule provides a semiempirical link between the conductivity prefactor and the activation energy
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