IP: 5.62.154.21 On: Thu, 06 Dec 2018 02:21:11 Copyright: American Scientific Publishers Delivered by Ingenta Copyright © 2019 American Scientific Publishers All rights reserved Printed in the United States of America Article Journal of Nanoscience and Nanotechnology Vol. 19, 1809–1813, 2019 www.aspbs.com/jnn Low-Temperature Sintering of Garnet-Type Li 7 La 3 Zr 2 O 12 Solid Electrolyte with Li 3 BO 3 Additive Prepared by Polymeric Complex Method Ran-Hee Shin and Sung-Soo Ryu ∗ Engineering Ceramic Center, Korea Institute of Ceramic Engineering and Technology, Icheon 17303, Korea In this study, a Li 3 BO 3 (LBO) compound is synthesized via the heat-treatment of polymeric precur- sors containing Li and B in air at 700  C for 5 h to use as a sintering additive for the densification of Li 7 La 3 Zr 2 O 12 (LLZ) solid electrolyte. The synthesized LBO powder is suitable for promoting the densification, cubic phase stability, and ionic conductivity of LLZ. X-ray diffraction analysis indicated that monophasic cubic LLZ could be obtained by the addition of LBO in sintering, changing to cubic LLZ phase from LZ impurities detected in raw LLZ. The sintered LLZ-12 wt% LBO showed that the densification of the LLZ with LBO occurred by a coupling effect including the particle rearrangement of LLZ in the melted LBO phase and grain growth of LLZ particles. The density of the LLZ-12 wt% LBO composite sintered at 1100  C for 8 h was 3.72 g/cm 3 (86% of theoretical density); the com- posite showed the high Li-ion conductivity of 1.18 × 10 -4 S · cm -1 at 28  C. Keywords: All-Solid-State Battery, Li Garnet Electrolytes, Sintering Additive, Li 3 BO 3 , Li-Ion Conductivity. 1. INTRODUCTION All-solid-state Li-ion secondary batteries with solid-state electrolytes have drawn significant attention for their excellent safety, reliability, and energy density. Solid-state electrolytes are superior to liquid electrolytes in various safety aspects, including suppressed formation of den- drites, flammability, and leakage. Therefore, all-solid-state Li-ion secondary batteries are becoming essential for many applications. 1 2 Many types of solid-state electrolytes, includ- ing perovskite titanate, 3–5 Na superionic conducting phosphates, 6–8 Li superionic conducting sulfides, 9 10 and garnet oxides 11–15 have been studied. Among these mate- rials, Li 7 La 3 Zr 2 O 12 (LLZ), with a cubic garnet structure, has a high ionic conductivity of 2 × 10 -4 S cm -1 at room temperature. Additionally, it exhibits excellent thermal and chemical stability against Li metal and commercial electrodes. 13 As a result, it has gained attention. To achieve high Li-ion conductivity, the cubic struc- ture of LLZ must be maintained, because the Li-ion con- ductivity of cubic-structured LLZ is greater than that of ∗ Author to whom correspondence should be addressed. tetragonal LLZ. To obtain the cubic structure, LLZ must be subjected to solid-state sintering at temperatures above 1200  C. However, the exposure to such high temperatures can cause Li to volatilize; therefore, cubic LLZ is first densified at a temperature below 1200  C. 16 For achiev- ing high Li-ion conductivity, this densification process is critical, because porous solid-state electrolytes exhibit low Li-ion conductivities and experience mechanical failure. 17 A liquid-phase sintering process is used to densify LLZ at low temperatures. In this process, a sintering additive, which has a low melting point and forms a liquid phase at temperatures below the solid-phase sintering temper- ature, is added. Li-based glass ceramics such as Li 2 O, Li 3 BO 3 (LBO), and LiSiO 4 are commonly used as sinter- ing additives for LLZ. 18–26 At temperatures below 1000  C, the glass ceramic sintering additive forms a liquid after melting. This liquid phase forms a thin layer at the grain boundaries of LLZ, thereby reducing the intergranular resistance and enhancing the total Li-ion conductivity. 18 According to Rosero-Navarro et al., 24 among the various Li-based glass ceramics, LBO is the most effective sinter- ing additive for the low-temperature densification of LLZ. J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 3 1533-4880/2019/19/1809/005 doi:10.1166/jnn.2019.16260 1809