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Functional Nanostructured Materials (including low-D carbon)
Multi-heteroatom-doped Hollow Carbon Attached on Graphene using LiFePO4 nanoparticles as Hard Templates for High-performance Lithium-sulfur Batteries Jeongyeon Lee, Jiseop Oh, Youngmoo Jeon, and Yuanzhe Piao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00925 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 13, 2018
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ACS Applied Materials & Interfaces
Multi-heteroatom-doped Hollow Carbon Attached on Graphene Using LiFePO4 Nanoparticles as Hard Templates for High-performance Lithium-sulfur Batteries Jeongyeon Lee,‡a Jiseop Oh,‡a Youngmoo Jeon,a Yuanzhe Piao*a,b
a
Program in Nano Science and Technology, Graduate School of Convergence Science and Technology, Seoul National University, 145 Gwanggyo-ro, Yeongtong-gu, Suwon–si, Gyeonggi-do, 443-270, Republic of Korea. b
Advanced Institutes of Convergence Technology, 145 Gwanggyo-ro, Yeongtong-gu, Suwon–si, Gyeonggi-do, 443-270, Republic of Korea.
*Corresponding author: Yuanzhe Piao:
[email protected] ‡
These authors contributed equally to this work
Keywords: heteroatoms, graphene, LiFePO4, cathodes, lithium-sulfur batteries
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Abstract P, O, and N heteroatoms-doped hollow carbons on graphene (PONHC/G) from nano-sized LiFePO4 (LFP) as a hard template are shown to be a very efficient sulfur host for lithium-sulfur (Li-S) batteries. The PONHC/G made from LFP nanoparticles as hard materials provides sufficient voids with various pore sizes for sulfur storage and doping of the carbon structures with various heteroatoms minimized dissolution/diffusion of the polysulfides. The obtained PONHC/G can store sulfur and mitigate diffusion of the dissolved polysulfide owing to the well-organized host structure and the strong chemical affinity for polysulfides due to the polarization effect of the heteroatom dopants. As a cathode, S@PONHC/G shows excellent cycle stability and rate capability, as confirmed by polysulfide adsorption analysis. Therefore, PONHC/G may show high potential as sulfur scaffold in the commercialization of Li-S batteries through additional modification and optimization of these host materials.
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1. Introduction Lithium-sulfur (Li-S) batteries are regarded as promising devices for energy storage applications with high energy density, such as smart electric grids and electric vehicles, due to their high theoretical specific capacity (1672 mA h g−1), inexpensiveness, and the natural abundance of sulfur1-5. However, in Li metal batteries, Li metal is considerably reactive and easy to form the lithium dendrites by the reaction with electrolytes, which happens to consume both the Li metal and the electrolyte during the continuous cycling6-7. Furthermore, in terms of cathode issues, commercialization of Li-S battery is hindered by the low Coulombic efficiency, poor real capacity, and fast capacity fading due to the dissolution and migration of lithium polysulfides (LiPSs) in the electrolyte8-11. Carbonaceous materials are promising as sulfur scaffolds for enhancing the electrical conductivity of the electrodes and trapping LiPSs by a physical barrier12-15. Thus, carbon materials with various pore types and structure have been used as host materials for sulfur16-20. Nazar's group synthesized a spherical ordered mesoporous carbon nanosphere/sulfur composite as a cathode material for Li-S batteries21. Shi and coworkers designed and synthesized an exfoliated reduced graphene oxide (RGO) coated carbon-sulfur composite22. Although these carbonaceous sulfur scaffolds improved the cycle performance of the Li-S batteries, severe capacity fading occurred after long-term cycling due to the low chemical affinity of the nonpolar carbon surface for the soluble LiPSs, where the carbonaceous materials have the limited ability as a physical barrier to prevent diffusion of the dissolved LiPSs. Thus, various hydrophilic metal-based oxides or sulfides, such as TiO223, 24, MnO225-27, Fe3O428, and FeS229 have been used as absorbents to mitigate the dissolution and diffusion of LiPSs by exploiting their strong chemical affinity for polarized metal-oxygen or sulfide bonds30, 31. Lou’s group reported that the carbon nanotubes 3 ACS Paragon Plus Environment
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(CNTs) filled with MnO2 sheets have been employed as sulfur hosts in Li-S batteries32, where improved electrochemical performance was derived from the synergetic effect of the carbon shell as a physical barrier and efficient confinement of the LiPSs via chemical adsorption by the MnO2 sheets. Zhang’s group indicated that oxygen-deficient TiO2 nanosheets were used as host materials for Li-S batteries, leading to enhanced performance by minimizing dissolution of the LiPSs due to good chemical bonding between the LiPSs and TiO233. The carbonaceous scaffolds can alternatively be functionalized by doping with heteroatoms, such as N34, 35, B36, P37, and O38. Heteroatom-doped carbon materials have been widely utilized as sulfur hosts for improving the electrochemical performance of Li-S batteries. Heteroatom doping in scaffolds mitigates diffusion of the soluble PSs and dissolved LiPSs via strong chemical adsorption, and is favorable for the high utilization of sulfur in the active material for Li-S batteries39-41. Besides, the high loading of sulfur cathode in electrode has been importantly addressed for commercialization, therefore, it is highly important to design the long cycle life and excellent rate capability, as well as the high loading in sulfur cathodes42. Herein, we choose nano-sized LiFePO4 nanoparticles (LFP) as templates to form an exceptional multi-heteroatom-doped carbon structure with numerous meso/macro pores as a highly effective sulfur host. The nano-sized LFP is uniformly coated with dopamine, which prevents agglomeration and maintains the original shape of the nitrogen-doped carbon shells after carbonization. The material is doped with P and O, where heating is applied for P and O doping within the templates. Using LFPs as hard templates, for the first time, we design and fabricate triple heteroatom (P, O, and N)-doped hollow carbon-on-graphene nanosheets (PONHC/G) and apply this material as a highly effective sulfur host in Li-S batteries. The asobtained PONHC/G has remarkable advantages: (1) the PONHC/G scaffold has a considerably high surface area (655.2 m2 g−1), resulting in high utilization of sulfur in its numerous meso/macro pores. (2) Owing to the triple doping with phosphorous, oxygen, and 4 ACS Paragon Plus Environment
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nitrogen, the polarized carbon scaffold possesses sufficient active sites (on the surface of PONHC/G), which strengthens the confinement of sulfur and LiPSs by potent chemical adsorption. (3) The graphene nanosheets are well distributed between the PONHC nanoparticles, and the PONHC/G composites aggregate, resulting in superior electrical conductivity. These synergistic effects lead to considerably stable cycle performance and excellent rate capability of the S@PONHC/G electrode.
2. Experimental section 2.1 Preparation of rGO Graphene oxide (GO) was prepared by the modified Hummers’ method43. Briefly, 3 g of graphite (Aldrich,
