Anomalies in Bulk Ion Transport in the Solid Solutions of Li 7 La 3 M 2 O 12 (M = Hf, Sn) and Li 5 La 3 Ta 2 O 12 .

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.
Competing Interests: The authors declare no competing financial interest.
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