FULL PAPER © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 6) 1400523 wileyonlinelibrary.com Dr. F. Lin, [+] Dr. D. T. Gillaspie, K. M. Jones, Dr. A. C. Dillon, Dr. C. Engtrakul National Renewable Energy Laboratory Golden, CO 80401, USA E-mail: chaiwat.engtrakul@nrel.gov Dr. F. Lin, Prof. R. M. Richards Materials Science Program Department of Chemistry and Geochemistry Colorado School of Mines Golden, CO 80401, USA Dr. D. Nordlund, Dr. T.-C. Weng Stanford Synchrotron Radiation Lightsource at SLAC Menlo Park, CA 94025, USA Dr. R. G. Moore Stanford Institute for Materials and Energy Science at SLAC Menlo Park, CA 94025, USA [+] Present address: Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Solid-State Conversion Reaction to Enhance Charge Transfer in Electrochromic Materials Feng Lin, Dennis Nordlund, Tsu-Chien Weng, Rob G. Moore, Dane T. Gillaspie, Kim M. Jones, Anne C. Dillon, Ryan M. Richards, and Chaiwat Engtrakul* DOI: 10.1002/admi.201400523 tiple efforts have been made to facilitate charge transfer, such as conformal inter- face layers, [9] electrode/electrolyte com- positional control, [15] and buffer layers. [8] However, these modification methods create additional interfaces, such as elec- trolyte/buffer layer and electrode/buffer layer interfaces. To date, a limited number of examples exist in which a continuous interlayer between the active electrode and electrolyte layer is utilized to improve the electrochemical properties of the active material. [16] The majority of these studies have been conducted to attempt to address current limitations in lithium-ion batteries. There are many similarities between thin film lithium-ion batteries and electrochromic materials. [17] Elec- trochromic transition metal oxide materials are suitable for high performance smart windows to improve energy efficiency for buildings, as well as to create thermally and visually comfort- able environments. [18–20] State-of-the-art electrochromic devices are configured using a multilayered device structure: Glass/ TCO/cathodic electrode/solid-state electrolyte/anodic electrode/ TCO/Glass , where TCO represents transparent conductive oxide ( Scheme 1A). [21,22] High quality electrochromic devices are not only reliant on the optimization of active layers [23,24] but also affected by the charge transfer across the representative interfaces. Recently, Lin et al. demonstrated that a high perfor- mance anodic electrode layer (i.e., Li 2.34 NiZr 0.28 O x ) has a sur- face enriched with lithium peroxide (Li 2 O 2 ) that forms during a cosputtering deposition process. It was suggested that the pres- ence of Li 2 O 2 was beneficial for enhancing the electrochromic switching properties (i.e., charge transfer) of the anodic elec- trode layer. [19] Theoretical investigations have shown that lithium vacancies [25,26] and carbon sheet dopants [27] can increase the hole charge carrier concentration in Li 2 O 2 . Therefore, an internal network of Li 2 O 2 has the potential to facilitate electron transport and hence improve the switching kinetics in lithium insertion materials. In light of this understanding, we herein report on the functionality of Li 2 O 2 as an interlayer between electrode and electrolyte while focusing specially on the anodic electrochromic electrode/electrolyte interface. To the best of our knowledge, this is the first attempt to modify the interface between an elec- trochromic film and a solid-state electrolyte. Two multilayered configurations were fabricated [Glass/TCO/Li 2.34 NiZr 0.28 O x / LiAlF 4 (Scheme 1B) and Glass/TCO/Li 2.34 NiZr 0.28 O x /Li 2 O 2 / LiAlF 4 (Scheme 1C)] to demonstrate the advantages of a con- tinuous Li 2 O 2 interlayer, where Li 2.34 NiZr 0.28 O x and LiAlF 4 films Interface engineering has attracted great interest and is essential for the fabrication of thin-film devices, such as smart windows. In this study, a solid-state conversion reaction for the development of an interlayer enriched with lithium peroxide (Li 2 O 2 ) is presented for an electrochromic device. We demonstrate that efficient lithium insertion and electron transport can be achieved by the inclusion of a Li 2 O 2 -rich interlayer between an active elec- trochromic material and Li ion solid-state electrolyte layer. The presence of a Li 2 O 2 -rich interlayer enhances electrochromic efficiency, kinetics, optical con- trast, and bleached-state transparency in a nickel oxide-based electrochromic thin film. This work opens up new opportunities to enhance the functionali- ties of thin-film devices by solid-state conversion reactions. 1. Introduction The chemistry and physics of lithium insertion solids have stim- ulated extensive studies in materials science [1,2] impacting many energy technologies such as lithium batteries and electrically tintable glass coatings. [3–6] Solid-state devices that accommodate lithium insertion reactions have attracted tremendous atten- tion due to their ease of fabrication and operation, negligible self-discharge rate, and enhanced cyclability. [7–11] The operation of lithium insertion cells involves the shuttling of lithium ions across the electrode/electrolyte interface, which is the controlling factor for charge transfer and governs the cycling rate [8,12,13] and switching kinetics [14] in lithium insertion cells. Mul- Adv. Mater. Interfaces 2015, 2, 1400523 www.advmatinterfaces.de www.MaterialsViews.com