Research Article www.acsami.org
Dimensional Effects of MoS2 Nanoplates Embedded in Carbon Nanofibers for Bifunctional Li and Na Insertion and Conversion Reactions Ji-Won Jung,† Won-Hee Ryu,‡ Sunmoon Yu,† Chanhoon Kim,† Su-Ho Cho,† and Il-Doo Kim*,† †
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Chemical and Biological Engineering, Sookmyung Women’s University, 100 Cheongpa-ro 47-gil, Yongsan-gu, Seoul, 04310, Republic of Korea S Supporting Information *
ABSTRACT: Controlling structural and morphological features of molybdenum disulfide (MoS2) nanoplates determines anode reaction performance for Li-ion and Na-ion batteries. In this work, we investigate dimensional effects of MoS2 nanoplates randomly embedded in twisted mesoporous carbon nanofibers (MoS2@ MCNFs) on Li and Na storage properties. Considering dimensions of the MoS2 nanoplates (e.g., interlayer, lateral distance, and slabs of stacking in number), we controlled thermolysis temperature to synthesize the MoS2 nanoplates with different geometry and optimize them in the hybrid anode for delivering high performance. The MoS2@MCNFs electrode exhibits reversible Li and Na capacities greater than 1000 cycles even at high current density of 1.0 A g−1 (1221.94 mAh g−1 with capacity retention of 95.6% for Li-ion batteries and 447.29 mAh g−1 with capacity retention of 87.11% for Na-ion batteries). We elucidated the insertion, conversion, and interfacial reaction characteristics of the thermosensitive MoS2 nanoplates in the MCNFs, especially associated with a reversible capacity. Our study will hint at rational design of the nanostructured MoS2 electrodes and focus on significance of their dimensional effects on anode performance. KEYWORDS: molybdenum disulfide, mesoporous nanofiber, electrospinning, lithium-ion batteries, sodium-ion batteries
■
or Na+ via insertion and subsequent conversion reactions [metallic Mo domains in M2S (M = Li or Na) as the final product].9,10 However, the specific capacities in excess of 670 mAh g−1 have often been found in the MoS2-based LIBs and NIBs. This can be explained by “capacitive behavior” of Mo nanoparticles (NPs).11−14 Nanostructured MoS2 provides capacitive sites for physical absorptions of Li+ or Na+ on the metallic Mo surfaces, which is derived after conversion reaction, contributing to the excess charge storages at low initial potential. The possible three reactions comprising insertion, conversion, and interfacial reactions, respectively, highly depend on dimensional properties of MoS2.15−18 Enlarging the interlayer spaces of MoS2 has often been considered as a common solution to facilitate insertion of Li or Na ions and consequently increase a discharge capacity in LIBs and NIBs.19−21 The enlarged interlayers of MoS2 are in favor of
INTRODUCTION With the increase of renewable energy demand, recent research trends have searched for efficient and sustainable energy storage systems to effectively manage the energy generated. Rechargeable alkali-ion batteries such as Li-ion batteries (LIBs) and Naion batteries (NIBs) have been considered as alternative candidates for the future energy devices because of their high energy density, excellent reversibility, and low cost. The performance of the batteries is mainly determined by introducing suitable electrode materials with a desirable structure affording reversible reaction chemistry.1,2 Molybdenum disulfide (MoS2), a family of chalcogenides, has been proposed as an analogous potential anode for both of LIBs and NIBs.3−6 The Mo and S are covalently bonded to form two-dimensional (2D) layers, which are stacked along the c-direction by weak van der Waals interaction between MoS2 layers. The neighboring layers with large interlayer spaces of 0.615 nm are sufficient to allow facile insertion and extraction of Li+ and Na+ without remarkable volume changes, giving rise to excellent rate capability and stable cyclability.7,8 The MoS2 can deliver a high theoretical capacity of 670 mAh g−1 with Li+ © 2016 American Chemical Society
Received: July 7, 2016 Accepted: September 21, 2016 Published: September 21, 2016 26758
DOI: 10.1021/acsami.6b08267 ACS Appl. Mater. Interfaces 2016, 8, 26758−26768
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustrations of (a) the MoS2@MCNFs and (b) electrochemical reaction mechanisms of the thermosensitive MoS2@MCNFs with Li+ or Na+.
embedded in highly mesoporous carbon nanofibers (MoS2@ MCNFs) via electrospinning and precisely controlled subsequent calcination temperatures. For the thermal treatment, precursor components in the fibers are decomposed at the beginning (
