One-Dimensional Peptide Nanostructure Templated Growth of Iron Phosphate Nanostructures for Lithium-Ion Battery Cathodes Hepi Hari Susapto, §,† O. Ulas Kudu, † Ruslan Garifullin, †,‡ Eda Yılmaz,* ,† and Mustafa O. Guler* ,† † Institute of Materials Science and Nanotechnology, National Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey ‡ Kazan Federal University, Institute of Fundamental Medicine and Biology, 18 Kremlyovskaya St., 420008 Kazan, Russian Federation * S Supporting Information ABSTRACT: Template-directed synthesis of nanomaterials can provide benefits such as small crystalline size, high surface area, large surface-to-volume ratio, and structural stability. These properties are important for shorter distance in ion/electron movement and better electrode surface/electrolyte contact for energy storage applications. Here nanostructured FePO 4 cathode materials were synthesized by using peptide nanostructures as a template inspired by biomineralization process. The amorphous, high surface area FePO 4 nanostructures were utilized as a cathode for lithium-ion batteries. Discharge capacity of 155 mAh/g was achieved at C/20 current rate. The superior properties of biotemplated and nanostructured amorphous FePO 4 are shown compared to template-free crystalline FePO 4 . KEYWORDS: peptide amphiphile, self-assembly, hydrogel, nanofiber, nanobelt, template-directed materials ■ INTRODUCTION High demand for portable electronic devices and electric and hybrid vehicles has increased the need for secondary batteries in the past decade. Lithium-ion batteries (LIBs) are currently the most prevalent secondary battery systems due to their high energy density, high voltage, light weight, and long cycle life. 1-4 Selection and synthesis of the electrode materials are very crucial to utilize these properties appropriately. Lithium transition metal oxides (LiMO x , M is a transition metal) are conventionally used as cathode materials for LIBs; however, there are some safety concerns associated with these materials, because they release oxygen from the lattice at high temperatures. 5 To increase the safety of cathode materials, materials with polyanion groups have been investigated. 6-8 On the one hand, olivine lithium iron phosphate (LiFePO 4 ) has been studied as cathode material due to its high stability and thermal safety, good theoretical capacity (∼170 mAh/g), and low material cost. 9-11 On the other hand, olivine structure has slow Li + ion and electron transfer, which leads to a poor rate capability. 12,13 Alternatively, amorphous iron phosphate (FePO 4 ) has received increasing attention because it is stable, safe, cheaper, and has a slightly higher theoretical capacity (∼178 mAh/g). 14 Moreover, it has a continuous charge-discharge voltage profile, which makes it easier to monitor the state of charge of the battery. 4 However, as amorphous FePO 4 suffers from low ionic and electronic conductivity, its practical capacity is dramatically lower than the theoretical value. 15,16 Several attempts have been shown to be effective such as nanostructures, 17-19 carbon nanopainting, 20 and conductive additives. 21,22 By using template-directed synthesis methods, nanostructures with desired shape, size, and function can be achieved. 23-25 These materials usually have small crystalline size, high surface area, large surface-to-volume ratio, and favorable structural stability, which result in shorter distance for ion/electron movement and better electrode surface/ electrolyte contact. 26 All these properties eventually lead to higher overall capacity, rate capability, and battery life. Biotemplating is an efficient way to synthesize environ- mentally friendly, ordered, and reproducible materials. 27 Biomineralization process was previously utilized for reaching these targets. 27,28 Previously, crystalline FePO 4 hollow nano- spheres with diameters of 7 μm were produced by using rape pollen grains as a template, but their electrochemical perform- ance was not studied. 27 Amorphous FePO 4 nanowires with diameters of 10 to 20 nm were synthesized by using genetically engineered M13 virus, showing an initial specific capacity of 100 mAh/g at a current rate of C/10. A heterostructured cathode material mixed with AgCl was synthesized by using biotemplat- ing approach, and their discharge capacity was increased to 150 mAh/g after cycling at the same current rate. 28 In this work, we show synthesis of nanobelt- and nanotube- shaped iron phosphate (FePO 4 ) nanostructures by using peptide nanostructures as biotemplates. By exploiting the fascinating features of the amphiphilic peptide molecules such as ease of modification over the chemical functionality and architecture diversity, 29 the nanonetworks were uniformly coated with a thin FePO 4 (∼8 nm) layer allowing the Li + ions to intercalate through Received: February 29, 2016 Accepted: June 17, 2016 Published: June 17, 2016 Research Article www.acsami.org © 2016 American Chemical Society 17421 DOI: 10.1021/acsami.6b02528 ACS Appl. Mater. Interfaces 2016, 8, 17421-17427 Downloaded via BILKENT UNIV on December 23, 2018 at 11:07:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.