The Electronic Conductivity of Single Crystalline Ga‐Stabilized Cubic Li₇La₃Zr₂O₁₂: A Technologically Relevant Parameter for All‐Solid‐State Batteries

Abstract

The next‐generation of all‐solid‐state lithium batteries need ceramic electrolytes with very high ionic conductivities. At the same time a negligible electronic conductivity σeon is required to eliminate self‐discharge in such systems. A non‐negligible electronic conductivity may also promote the unintentional formation of Li dendrites, being currently one of the key issues hindering the development of long‐lasting all‐solid‐state batteries. This interplay is suggested recently for garnet‐type Li7La3Zr2O12 (LLZO). It is, however, well known that the overall macroscopic electronic conductivity may be governed by a range of extrinsic factors such as impurities, chemical inhomogeneities, grain boundaries, morphology, and size effects. Here, advantage of Czochralski‐grown single crystals, which offer the unique opportunity to evaluate intrinsic properties of a chemically homogeneous matrix, is taken to measure the electronic conductivity σeon. Via long‐time, high‐precision potentiostatic polarization experiments an upper limit of σeon in the order of 5 × 10−10 S cm−1 (293 K) is estimated. This value is by six orders of magnitude lower than the corresponding total conductivity σtotal = 10−3 S cm−1 of Ga‐LLZO. Thus, it is concluded that the high values of σeon recently reported for similar systems do not necessarily mirror intragrain bulk properties of chemically homogenous systems but may originate from chemically inhomogeneous interfacial areas.

The Devil is in the Defects: Electronic Conductivity in Solid Electrolytes

“…13 Dendrites were not observed in the case of LiPON, 13 with s e values (10 15 -10 12 S/cm) at least four orders of magnitude lower than those in LLZO and Li 3 PS 4 . In striking contrast, recently, Philipp et al 14 reported s e of ⇠10 10 S/cm at 293 K in single-crystal Ga-doped LLZO, and claimed that such "low" s e cannot be responsible for Li dendrite growth in LLZO. These evidences point to a lack of consensus on the effects of s e in SEs.…”

“…It has been demonstrated that electrolyte decomposition products at both high and low voltages (vs. Li/Li + ) may display intrinsic electronic conductivity, 32,33,38,46 whose magnitudes remain elusive. 13,14,45 While s e values are typically reported for completeness, the focus of many reports (e.g., references in Figure 1) shifts entirely to the high intrinsic Li + -ionic conductivity of novel SE chemistries. Solid electrolytes that are oxides, sulfides, or selenides, and even phosphates and silicates, 25 typically display band gaps >4 eV as seen in Figure 1.…”

Understanding metal propagation in solid electrolytes due to mixed ionic-electronic conduction

“…In addition, the electronic conductivity of some inorganic ceramic SEs has been shown to increase rapidly under high voltage. 21,22 Work with density functional theory has shown that enhanced electronic conductivity can originate from point defects 23 and internal pores and crack surfaces in the SE. 24 To systematically study the electronic conductivity-driven metal propagation in the SE, one must consider the SE as a mixed ionic-electronic conductor (MIEC).…”

Low Electronic Conductivity of Li7La3Zr2O12 (LLZO) Solid Electrolytes from First Principles

“…This result is consistent with recent experimental data that gave a much lower electronic conductivity for single-crystal samples of LLZO than previously reported for polycrystalline samples. 55 Our analysis presented here exclusively considers bulk defect populations and their response to doping, and therefore we cannot exclude the possibility that other sources of free charge-carriers might facilitate direct inplace reduction of lithium ions to lithium metal. Previous theoretical work has observed dramatic band-gap reductions at the surfaces of LLZO (E bulk g = 5.46 eV, E surface g = 2.19 eV).…”