Article pubs.acs.org/JPCC
Borophene as Efficient Sulfur Hosts for Lithium−Sulfur Batteries: Suppressing Shuttle Effect and Improving Conductivity Lin Zhang,† Pei Liang,*,† Hai-bo Shu,† Xiao-lei Man,† Feng Li,† Jie Huang,†,§ Qian-min Dong,† and Dong-liang Chao‡ †
College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore § College of Optical Electrical and Computer Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China ‡
S Supporting Information *
ABSTRACT: Stabilizing polysulfide shuttle while ensuring high sulfur loading holds the key to realizing high energy density of lithium−sulfur (Li−S) batteries. Herein we present our first-principles calculation on borophene as host of cathode in Li−S battery. The adsorption energies of sulfur cluster (S8) and its discharge products (Li2S8, Li2S6, Li2S4, Li2S2, and Li2S) on borophene are calculated. Our results indicate that borophene host can trap lithium polysulfides stably and effectively, which could avoid shuttle effect and improve the utilization of active material. The band structure of the adsorption structures shows that the borophene−sulfur cluster is metallic, while partial charge density proves that the conductivity is mainly due to the metallic borophene substrate. Such favorable electrical conductivity is helpful to the cathode charge/discharge processes. Therefore, borophene could be a promising host for S cathode due to its strong adsorption, high conductivity, and small deformation. carbon−sulfur composites,8 conductive polymer−sulfur composites,9,10 metal oxide,11,12 and sulfide coating. Moreover, various routes of physically trapping soluble polysulfides have been explored, including the insertion of a microporous carbon paper between cathode and separator and modification of separators with a carbon coating layer. The weak interaction with the polar Li2Sn undermines their application as polysulfide traps; even carbon-based materials have shown promise as sulfur-entrapping host structures. Two-dimensional (2D) materials are believed to be a good choice to overcome the problems mentioned above due to their high surface−volume ratio and unique electronic properties by comparing with bulk counterparts.13,14 Today, the 2D materials, including graphene,15−17 silicone,18,19 phosphorene,7,20 oxides,21 sulfides (such as MoS2),6 and so on, have been extensively used in energy storage and conversion devices. Very recently, borophene (2D boron sheet), as a new type 2D material, has attracted tremendous interest due to its outstanding properties.22 Boron is the fifth element in the periodic table, which has sp2 hybrid orbit similar to carbon.
I. INTRODUCTION With increasing demand for high capacity energy storage in secondary batteries, many efforts have been focused on the next generation of secondary batteries following lithium-ion battery (LIB). Lithium−sulfur (Li−S) battery has been considered a new candidate owing to its high theoretical specific energy of 2600 Wh·kg−1 (on account of a capacity of 1673 mAh·g−1) that is over six time higher than that of currently used commercial LIB (387 Wh·kg−1 for LiCoO2/C battery).1,2 Furthermore, the low cost and toxicity of sulfur makes it more attractive for commercial applications.3,4 Although Li−S batteries have so many advantages, they are not widely used in commerce because of their short cycle life, poor safety, high self-discharge rate, low active material utilization, and so on.5 All of these shortages are mainly attributed to the following reasons: (1) the dissolution of lithium polysulfides, which causes seriously shuttle effect; (2) severe volume expansion of sulfur upon cycling; and (3) poor conductivity of sulfur, Li2S, and Li2S2.4,6,7 To address these issues, extensive research was devoted to the design of the electrode structure and composition to increase the conductivity and prevent the polysulfide’s dissolution by physically or chemically trapping the sulfur species within the electrodes. To date, several important strategies for advanced electrode design have been explored such as nanoporous © 2017 American Chemical Society
Received: April 20, 2017 Revised: June 28, 2017 Published: June 30, 2017 15549
DOI: 10.1021/acs.jpcc.7b03741 J. Phys. Chem. C 2017, 121, 15549−15555
Article
The Journal of Physical Chemistry C Boron also has some charming natures such as super hardness,23−27 good conductivity,28−30 thermoelectricity31,32 and high chemical stability.32,33 The B−B bond in boron materials is very strong and has complex delocalization. For Li− S batteries, the outstanding integrate properties of borophene may solve the key problems, such as stable polysulfide shuttle of polysulfides, avoid volume expansion of sulfur, and improve conductivity. Most recently, Feng et al. have obtained two monolayer boron structures (β12 phase and χ3 phase) on Ag(111) substrate by using ultrahigh vacuum molecular beam epitaxy (MBE).22 Mannix et al. ablated solid boron in an ultrahigh vacuum using an electron beam evaporator and successfully fabricated borophene with only one atom thickness on the silver surface. The successful synthesis of borophene provides the possibility of preparing borophene-based electronic devices. Because of numerous advantages and successful fabrication of borophene, the theoretical34,35 and experimental study22,36 of borophene has become a hotspot recently, especially borophene as a battery material. Several theoretical studies37−40 have shown that borophene is a very promising electrode material for ion battery because of its extremely high power density and rate capability. However, there are seldom studies on the borophene used as an efficient sulfur host material for Li−S battery. The extensive exploration of these novel nanostructures to be effective anchor Li2Sn species for better Li−S cathode inspires us to ask an interesting question: Can borophene be utilized as a good host material for Li−S battery? To answer this question, we investigated the adsorption of various Li2Sn intermediates at different lithiation/delithiation states on borophene by means of comprehensive density functional theory (DFT) computations. Our results revealed that Li2Sn species could interact with borophene with the adsorption energy ranging from −1.00 to −3.00 eV, suggesting that borophene is an excellent anchoring material. Because of the metallic properties and the monolayer structure of borophene, the hybrid material generally shows metallic, resulting in an improved electrical conductivity. Thus borophene is deemed as a promising anchoring material for Li2Sn species with high performance.
Figure 1. (a) Top view of fully optimized structure of the borophene surface in a 5*3 supercell and fully optimized molecular structures of isolated (b−f) Li2Sn (n = 1, 2, 4, 6, 8) and (g) S8 clusters in the ground states, respectively. Here yellow and green balls symbolize sulfur and lithium atoms, respectively.
all of the forces on each atom were
