Stable Interface Formation between TiS 2 and LiBH 4 in Bulk-Type All- Solid-State Lithium Batteries Atsushi Unemoto,* ,† Tamio Ikeshoji, †,‡ Syun Yasaku, ‡ Motoaki Matsuo, ‡ Vitalie Stavila, § Terrence J. Udovic, ∥ and Shin-ichi Orimo †,‡ † WPI−Advanced Institute for Materials Research (WPI−AIMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan § Energy Nanomaterials, Sandia National Laboratories, Livermore, California 94551, United States ∥ NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6102, United States * S Supporting Information ABSTRACT: In this study, we assembled a bulk-type all-solid-state battery comprised of a TiS 2 positive electrode, LiBH 4 electrolyte, and Li negative electrode. Our battery retained high capacity over 300 discharge−charge cycles when operated at 393 K and 0.2 C. The second discharge capacity was as high as 205 mAh g −1 , corresponding to a TiS 2 utilization ratio of 85%. The 300th discharge capacity remained as high as 180 mAh g −1 with nearly 100% Coulombic efficiency from the second cycle. Negligible impact of the exposure of LiBH 4 to atmospheric-pressure oxygen on battery cycle life was also confirmed. To investigate the origin of the cycle durability for this bulk-type all-solid-state TiS 2 /Li battery, electrochemical measurements, thermogravimetry coupled with gas composition analysis, powder X-ray diffraction measurements, and first-principles molecular dynamics simulations were carried out. Chemical and/or electrochemical oxidation of LiBH 4 occurred at the TiS 2 surface at the battery operating temperature of 393 K and/or during the initial charge. During this oxidation reaction of LiBH 4 with hydrogen (H 2 ) release just beneath the TiS 2 surface, a third phase, likely including Li 2 B 12 H 12 , precipitated at the interface between LiBH 4 and TiS 2 . Li 2 B 12 H 12 has a lithium ionic conductivity of log(σ / S cm −1 )= −4.4, charge transfer reactivity with Li electrodes, and superior oxidative stability to LiBH 4 , and thereby can act as a stable interface that enables numerous discharge−charge cycles. Our results strongly suggest that the creation of such a stable interfacial layer is due to the propensity of forming highly stable, hydrogen-deficient polyhydro-closo-polyborates such as Li 2 B 12 H 12 , which are thermodynamically available in the ternary Li−B−H system. 1. INTRODUCTION The all-solid-state battery, which consists of solid-state components (anode, cathode, and electrolyte), is considered as one of the most promising candidates for future-generation energy storage. 1,2 This is because the solid-state electrolytes used in these batteries expand the choice of the electrodes incorporated into the battery and allow for flexible battery design, i.e., bipolar stacking structure, which is advantageous in terms of both energy and power densities. 3 In addition, it overcomes the concerns related to safety, including Li dendrite formation and leakage and vaporization of liquid electrolytes, currently problematic for the commercial lithium-ion batteries that use organic liquid solvents. 1,2 Thus, the all-solid-state battery would be advantageous for utilization in large-scale applications including stationary uses for load leveling, electric vehicles, and so forth. Research and development efforts of solid-state electrolytes for all-solid-state batteries have so far been focused mainly on oxides and sul fides, some of which have fast ionic conductivities. 4 Besides favorable conductivities, it is also crucial for durable, high-performance battery operation to possess an interface with both high electrochemical and chemical stabilities. It has been suggested that the mutual diffusion of constituent elements across the interface between the positive electrode and electrolyte increases the interface resistance, resulting in capacity fading. 5,6 In such systems, introduction of a protective layer is effective for enhancing the cycle life. Thus, the overall success of the all-solid-state battery relies not only on realizing the fast ionic conduction of solid- Received: June 4, 2015 Revised: July 14, 2015 Published: July 14, 2015 Article pubs.acs.org/cm © 2015 American Chemical Society 5407 DOI: 10.1021/acs.chemmater.5b02110 Chem. Mater. 2015, 27, 5407−5416