Thin and Dense Solid-solid Heterojunction Formation Promoted by Crystal Growth in Flux on a Substrate

Nobuyuki Zettsu,Kunio Yubuta,Katsuya Teshima,Kazune Nemoto,Hiromasa Shiiba,Hitoshi Onodera,Takeshi Kimijima,Masanobu Nakayama

doi:10.1038/s41598-017-18250-9

Abstract

In this work, we demonstrate the direct growth of cubic Li5La3Nb2O12 crystal layer on the LiCoO2 substrate through the conversion of ultra-thin Nb substrate in molten LiOH flux. The initial thickness of the Nb layer determines that of the crystal layer. SEM and TEM observations reveal that the surface is densely covered with well-defined polyhedral crystals. Each crystal is connected to neighboring ones through the formation of tilted grain boundaries with Σ3 (2–1–1) = (1–21) symmetry which show small degradation in lithium ion conductivity comparing to that of bulk. Furthermore, the sub-phase formation at the interface is naturally mitigated during the growth since the formation of Nb2O5 thin film limits the whole reaction kinetics. Using the newly developed stacking approach for stacking solid electrolyte layer on the electrode layer, the grown crystal layer could be an ideal ceramic separator with a dense thin-interface for all-solid-state batteries.

Highlights

Various oxide electrolytes, such as perovskite-type Li3xLa2/3−x□1/3−2xTiO3 (LLTO), LISICON, and NASICON have been widely investigated
The Li5La3Nb2O12 crystal layers were prepared on a Nb substrate by the flux growth method using a LiOH flux, in which LiOH·H2O and La2O3 powders reacted with the Nb substrate
The conversion of Nb substrate in molten LiOH flux formed crystal layers with well-defined faces, and each individual crystal is connected to neighboring ones

Summary

Introduction

Various oxide electrolytes, such as perovskite-type Li3xLa2/3−x□1/3−2xTiO3 (LLTO), LISICON, and NASICON have been widely investigated. Even though it was expected that such solid electrolyte thin film provides low electrical resistance and increasing volumetric energy density for all-solid-state battery, their lithium ion conductivities at room temperature were in the range of 10−7–10−5 S·cm−1, which is nearly two digits lower than sintered pellets samples. These film fabrication methods are efficient for fabricating uniform film and controlling the film thickness, in some cases, it is difficult to control elementary composition and microstructures. Heterojunction formation between Li5La3Nb2O12 and LiCoO2 ceramics (which is the most popular cathode active material) was performed through a similar approach

Methods

Results

Conclusion