Hierarchical Carbon with High Nitrogen Doping Level: A Versatile

Subscriber access provided by The University of British Columbia Library

Article

Hierarchical Carbon with High Nitrogen Doping Level: A Versatile Anode and Cathode Host Material for Long-Life Lithium-Ion and Lithium-Sulfur Batteries Christian Reitz, Ben Breitung, Artur Schneider, Di Wang, Martin von der Lehr, Thomas Leichtweiß, Jürgen Janek, Horst Hahn, and Torsten Brezesinski ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12361 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Hierarchical Carbon with High Nitrogen Doping Level: A Versatile Anode and Cathode Host Material for Long-Life Lithium-Ion and LithiumSulfur Batteries Christian Reitz,*,†,‡ Ben Breitung,§ Artur Schneider,§ Di Wang,‡ Martin von der Lehr,# Thomas Leichtweiss,# Jürgen Janek,§,# Horst Hahn,†,+ and Torsten Brezesinski*,†,§ †

Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany.

‡

Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Hermann-von-HelmholtzPlatz 1, 76344 Eggenstein-Leopoldshafen, Germany.

§

Institute of Nanotechnology, Battery and Electrochemistry Laboratory, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. #

Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany.

+

Helmholtz Institute Ulm for Electrochemical Energy Storage, Helmholtzstr. 11, 89081 Ulm, Germany.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

KEYWORDS Hard templating, atomic layer deposition, rechargeable battery, N-doped carbon, sulfur, titania

ABSTRACT

Nitrogen-rich carbon with both a turbostratic microstructure and meso-/macroporosity was prepared by hard templating through pyrolysis of a tricyanomethanide-based ionic liquid in the voids of a silica monolith template. This multifunctional carbon is not only a promising anode candidate for long-life lithium-ion batteries, but also shows favorable properties as anode and cathode host material owing to a high nitrogen content (> 8% after carbonization at 900 °C). To demonstrate the latter, the hierarchical carbon was melt-infiltrated with sulfur as well as coated by atomic layer deposition (ALD) of anatase TiO2, both of which led to high quality nanocomposites. TiO2 ALD increased the specific capacity of the carbon while maintaining high Coulombic efficiency and cycle life – the composite exhibited stable performance in lithium half-cells, with excellent recovery of low rate capacities after thousands of cycles at 5C. Lithiumsulfur batteries using the sulfur/carbon composite also showed good cyclability, with reversible capacities of ~700 mAh g–1 at C/5 and without obvious decay over several hundreds of cycles. The present results demonstrate that nitrogen-rich carbon with an interconnected multimodal pore structure is very versatile and can be used both as active and inactive electrode material in high-performance lithium-based batteries.


2

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION The development of advanced battery technologies for stationary and transportation applications is of crucial importance, as this will help pave the way for a more sustainable use of renewable energy sources. Among the major challenges of next generation batteries are the need for improved performance (specific energy and power), energy efficiency and longevity; increased safety; and reduced costs. The advantages and disadvantages of various novel battery concepts, their cell reactions and active materials are described comprehensively elsewhere and thus, will not be discussed in detail.1–6 In the present work, we focus on nitrogen-rich carbon (referred to as N-doped carbon in the following) with an interconnected hierarchical pore structure for Li-based batteries. In recent years, N-doped carbons have been proven to be versatile materials in several fields. In addition to battery and supercapacitor applications, they hold promise, for example, as metal-free electrocatalysts and durable catalyst supports.7–15 The material employed here was prepared by silica templating using 1-ethyl-3-methylimidazolium tricyanomethanide (EMIM-TCM) as carbon precursor. As some other ionic liquids, EMIM-TCM is, as we will show, well suited for the preparation of carbon having both a turbostratic microstructure and high N doping level.16–18 The latter endows the carbon with a unique combination of physical properties (high electronic conductivity, enhanced adsorption capacity for lithium polysulfides etc.),16–20 which makes it not only a viable anode candidate, but also a promising host for different positive and negative electrode materials. Specifically, we show that atomic layer deposition (ALD) is a powerful means to chemically modify the hierarchical N-doped carbon for the construction of long-life batteries with highperformance stability.21 Anatase TiO2 was chosen as a model system, because it can be deposited


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

in nanocrystalline form at moderate temperatures,22 has been extensively studied in the past and is an alternative for graphite as anode material to achieve high-safety Li-ion batteries, in which specific energy is not the major concern.23,24 The primary reason is that the insertion/extraction potential of anatase TiO2, with a theoretical specific capacity of 168 mA h g–1, lies within the stability window of standard carbonate-based electrolytes. This means that the risk of both gas generation as a result of reductive electrolyte decomposition and Li metal plating is significantly reduced. However, we note that gassing of lithium titanate (Li4Ti5O12, LTO) electrodes has been recently reported and attributed to the presence of absorbed moisture in the cell.25,26 Moreover, we show that the hierarchical N-doped carbon can be utilized as effective sulfur host for stable Li-S batteries by combining physical confinement with chemical adsorption of lithium polysulfides. The Li-S system is currently one of the most researched battery technologies because of its high theoretical energy density of ~2500 Wh kg−1.27,28 However, the cell chemistry − involving the conversion of S8 to Li2S through formation of soluble lithium polysulfides as intermediate species − is complicated and inherently leads to performance issues such as fast capacity decay and low energy efficiency (due to polysulfide shuttle).29–33 It should be noted as well that S8 and Li2S are electrically insulating, which also adversely affects the performance, including sulfur utilization and charge/discharge kinetics. Thus, the use of a functional and conductive cathode host is essential to achieve Li-S batteries with long cycle life and good rate capability. Overall, in this paper we demonstrate that partially graphitic carbon with both a high N content and controlled meso-/macroporosity lends itself as high-performing active and inactive electrode material for multiple reasons: the functionality and electronic properties are significantly


4

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


improved as compared to N-free carbon; and the pore network is robust, provides void space for volume expansion and allows efficient electron transfer and good electrolyte penetration.

EXPERIMENTAL SECTION Preparation of Monolithic N-Doped Carbon. The carbon was prepared by hard templating using a hierarchically porous silica monolith template.34 The latter material was synthesized according to an established method based on spinodal decomposition of a poly(ethylene oxide)/tetramethyl orthosilicate mixture.35,36 The silica monoliths were immersed in 1-ethyl-3methylimidazolium tricyanomethanide (EMIM-TCM, IoLiTec) and full infiltration of the pore network was achieved under vacuum. This solid/liquid composite was then heated in an oven under argon to 900 °C over the course of 2 h. Infiltration and carbonization were repeated twice before removing the silica scaffold by leaching with a 3 M aqueous solution of potassium hydroxide at 70 °C. Preparation of Titania/N-Doped Carbon Composite. 1.5 g of N-doped carbon were ground in a mortar to a fine powder and then placed inside a fused silica glass container sealed at the top by a glass frit to ensure homogeneous gas transport. Atomic layer deposition (ALD) was carried out using an R-200 Advanced system from Picosun equipped with Picoflow diffusion enhancer. The latter allows increasing the retention time of the precursor in the reaction chamber by slowing down the pumping speed. As can be seen from Scheme 1, each of the two half-step reaction cycles consisted of injecting 14 times titanium tetrachloride or water into the chamber followed by a short purge period. After the 15th injection of reactant, the half-reaction was completed by a longer purge pulse of 35 s. The growth cycles were repeated 600 times using a hot-wall reactor at


5


250 °C with nitrogen as carrier gas. A silicon wafer inside the ALD reactor served as thickness reference. 4 s 0.2 s

20 s

5 s + 35 s th

Final Purge

Reduced Flow Purge

TiCl4 Pulse

15 Pulse

Reduced Flow

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

TiCl4 Retention x 14

Scheme 1. Schematic illustration of the half-step reaction cycle with TiCl4 as reactant. The same sequence was also used for H2O. Repetitive application of these half-reactions leads to a conformal anatase TiO2 coating on the free surface of the hierarchical N-doped carbon.

Preparation of Sulfur/N-Doped Carbon Composite. The composite was prepared by melt diffusion of elemental sulfur into the monolithic N-doped carbon followed by ball-milling for 1 h. The sulfur loading on the carbon was 77 wt.%. Lithium Polysulfide Adsorption Study. 50 mg of carbon were placed into a vial containing 5 mL of a 5 mM solution of Li2S6 in tetrahydrofuran. The vial was kept inside an argon-filled glovebox (MBraun) for 10 h to allow for equilibration. After carbon separation and dilution of the solution by a factor of 5, the adsorption capacity was determined from the change in absorption at 415 nm. Electrodes, Cell Assembly and Electrochemical Testing. Both the N-doped carbon and ALDderived titania/N-doped carbon electrodes were prepared by casting an N-ethyl-2-pyrrolidone


6

Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Sigma-Aldrich) slurry containing 85 wt.% active material, 10 wt.% carbon black (Super C65, Timcal) and 5 wt.% polyvinylidene fluoride (Kynar) onto copper foil (Gould Electronics) followed by drying in vacuum at 120 °C for 12 h. The titania and carbon loadings were on average 2.0 mg cm–2 and 2.5 mg cm–2, respectively. Coin cells with lithium metal anode (China Lithium Ltd., 600 µm) and glass microfiber film separator (Whatman Grade GF/D) were assembled inside an argon-filled glovebox. The electrolyte was 1 M LiPF6 in fluoroethylene carbonate and ethyl methyl carbonate (1:1 weight ratio). The cycling performance was evaluated at various rates ranging from C/2 to 20 C after activation at C/10 (with 1C = 168 mA gtitania–1 and 371 mA gcarbon–1, respectively). The sulfur cathodes were prepared by casting a water slurry containing 72 wt.% sulfur/Ndoped carbon, 18 wt.% carbon black (Super C65 and Printex XE2 (Orion), 1:1 weight ratio) and 10 wt.% poly(vinyl alcohol) (Selvol 425, Sekisui) onto carbon-coated aluminum foil followed by drying in vacuum at 60 °C for 12 h. The sulfur loading was about 2 mg cm–2. Coin cells consisting of sulfur cathode, lithium metal anode (China Lithium Ltd., 600 µm) and polyethylene film separator (SETELA, Toray) were assembled inside an argon-filled glovebox by using 0.325 M lithium bis(trifluoromethanesulfonyl)imide (Sigma-Aldrich) and 0.675 M lithium nitrate (Merck) in a 1:1 mixture of 1,2-dimethoxyethane (Alfa Aesar) and 1,3-dioxolane (Acros) as electrolyte. The electrolyte/sulfur ratio was set to 10:1. The cycling performance was evaluated at C/5 after activation at C/50 (with 1C = 1672 mA gsulfur–1). The charge cut-off potential was kept constant until a current drop of 90% was achieved before starting the next discharge cycle. Instrumentation. Transmission electron microscopy (TEM) imaging was performed using an aberration corrected Titan 80-300 (FEI) operated at 300 keV. Scanning electron microscopy (SEM) images were recorded on a MERLIN from Carl Zeiss at 5 keV. Powder X-ray diffraction


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

(XRD) was carried out on a STOE Stadi P using Cu Kα radiation. Rietveld refinement was performed with the FullProf software. The background was fitted by a linear interpolation between 28 background points with refinable height. NIST LaB6 660b was used as standard reference material. X-ray photoelectron spectroscopy (XPS) was performed using a VersaProbe PHI 5000 Scanning ESCA Microprobe from Physical Electronics equipped with Al Kα source. The C 1s signal at 284.8 eV for sp3 carbon served as energy reference to correct for charging. Nitrogen physisorption was carried out at T = 77 K on an Autosorb-6 automated gas adsorption station from Quantachrome Corporation. The pore size distribution was calculated using nonlocal density functional theory (NLDFT) equilibrium models for carbon (for N-doped carbon) or silica (for titania/N-doped carbon) assuming slit/cylindrical pore geometry. For mercury intrusion porosimetry, a setup from Thermo Fisher Scientific (Pascal 140/440 with pressure range from 0 to 400 MP) was used. Raman spectra were recorded on a SENTERRA dispersive Raman microscope from Bruker Optics equipped with YAG:Nd laser (532 nm, 2 mW) and Olympus objective (MPlan N 50×). Ultraviolet-visible (UV-Vis) spectroscopy was carried out on a Varian Cary 500 Scan UV-Vis-NIR spectrophotometer. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 F1 Libra. Electrochemical measurements were carried out at 25 °C in a BINDER cooled incubator using a MACCOR Series 4000 (Tulsa) multichannel battery cycler.

RESULTS AND DISCUSSION As mentioned in the introduction, the N-doped carbon employed in this work was made by pyrolysis of a tricyanomethanide-based ionic liquid in the voids of a hierarchically structured silica monolith template. It was utilized both as host for sulfur and titania and as an active anode


8

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


material in rechargeable Li-ion batteries. The corresponding nanocomposites were fabricated by facile melt diffusion of sulfur into the pore space and ALD of TiO2 onto the N-doped carbon using TiCl4 and H2O at 250 °C, and were found by TGA (Figure S1, Supporting Information) to contain 77 wt.% sulfur and 51 wt.% titania, respectively. The structure of the N-doped carbon and the composite material with TiO2 was investigated using electron microscopy. SEM micrographs at different magnifications are shown in Figures 1a, b and S2 (Supporting Information). Based on SEM, the as-prepared carbon exhibits an interconnected network of macro- and mesopores. This hierarchical architecture is retained after ALD of TiO2 and the pores are only partially filled, as is evident from Figure 1b. However, the surface structure changed considerably, which was expected given that both materials differ in their physical properties and forms. The Z-contrast high-angle annular dark-field scanning TEM (HAADF-STEM) micrograph of a 200 nm diameter composite particle in the inset of Figure 1b indicates that the ALD-derived TiO2 is uniformly distributed throughout the carbon. The conformal coating of the porous host is also confirmed by high-resolution TEM (HRTEM) and energy dispersive X-ray spectroscopy (EDS). HRTEM (Figure 1c) shows anatase TiO2 nanocrystals embedded in turbostratic carbon with an interlayer spacing of about 3.4 Å. The elemental maps and EDS spectrum in Figures 1d-g demonstrate the homogeneous distribution of C, O and Ti atoms across a large area and show that no major impurities are present in the TiO2/N-doped carbon composite. Both the phase composition and high crystallinity of TiO2 are further evidenced by selected-area electron diffraction (SAED). All the Debye-Scherrer rings in Figure 1h correspond to the anatase phase with space group I41/amd (see ICSD reference card no. 9852).37 In a nutshell, the electron microscopy data establish that the N-doped carbon can be uniformly coated using ALD. The resulting composite material has a unique structure and


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

consists of a ~1:1 mixture of multimodal carbon with partially graphitic pore walls and single phase anatase TiO2 nanocrystals of diameter

Hierarchical Carbon with High Nitrogen Doping Level: A Versatile

Recommend Documents