Aug 19, 2015 - INTRODUCTION. The lithiumâsulfur (Li/S) battery has been widely accepted as a promising alternative power source for portable electronics.
Article pubs.acs.org/JPCC
Anchoring Lithium Polysulfides via Affinitive Interactions: Electrostatic Attraction, Hydrogen Bonding, or in Parallel? Zhuan Ji, Bo Han,* Qiyang Li, Chenggang Zhou,* Qiang Gao, Kaisheng Xia, and Jinping Wu Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, Hubei China P.R. S Supporting Information *
ABSTRACT: Stabilizing lithium polysulfides in cathodes via interactions between polysulfides and affinitive functional groups could prevent polysulfide dissolution, leading to suppressed “shuttle effect” of lithium/sulfur (Li/S) batteries. Herein, four deoxynucleotides (DNs), including A (adenine-DN), T (thymine-DN), G (guanineDN), and C (cytosine-DN), which own rich polysulfide affinitive groups, are selected to model the anchoring environments of polysulfides. Using the most soluble Li2S8 as probe, our first-principles simulations suggest that the interactions between polysulfides and substrates are highly correlated to the charges of affinitive sites, H-bonding environments and structural tension. The contributions from each type of interactions are quasiquantitatively assessed. The electrostatic attractions between Li+ and the strong electron lone-pairs dominate the adsorption energetics, while the H-bonds formed between S82− and substrate give rise to excessive stabilization. In contrast, structural distortion or rearrangement of the substrates is detrimental to the anchoring strengths. The quasi-quantitative resolution on the different interaction modes provides a facile and rational scheme for screening more efficient polysufide affinitive additives to sustain the cathode cyclicity of Li/S batteries.
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composite or S either as protecting shells32−36 or as cathode additives.37−43 For example, the available discharge capacity of the poly(ethylene glycol) (PEG) modified CMK-3/S composite reported by Nazar et al.44 is averagely 300 mAh g−1 higher than the unmodified CMK-3/S. Alternatively, metal oxides such as TiO245 or MnO241 serving as additives have also been proved to be capable of entrapping polysulfides for improving capacity sustainability. In general, for most carbon substrates, there should exist a certain amount of functional groups such as −OH species, which could also be observed on the surfaces of metal oxide particles. For the reported polymer additives or coats, other types of functional groups including −NHx and −CO may appear. Without exception, these functional groups all own strong lone-pair electrons, which should be affinitive to the positively charged ions, in particular, the Li+ ions at the cathode/electrolyte interfaces in a Li/S system. This fact has been confirmed by the theoretical simulations conducted by Cui et al.,33,46 where −NHx, −CO and halogen groups were calculated. These groups, especially carbonyl groups, has intensive interaction with Li2S and Li−S• (represents long chain polysulfides). Upon such understandings, they sequentially proposed poly(vinylpyrrolidone) (PVP) as a bifunctional binder which significantly improves the cyclic performance of Li/S batteries comparing with conventional polyvinylidene fluoride (PVDF) binder.
INTRODUCTION The lithium−sulfur (Li/S) battery has been widely accepted as a promising alternative power source for portable electronics and particularly for electric vehicles due to its high theoretical energy density of 2600 Wh kg−1. In addition, sulfur is abundant in nature, low cost, and low toxicity. However, successful deployment of Li−S batteries is suffered by the low conductivity of active materials, large volume expansion during lithiation, and in particular, the rapid capacity fading caused by the “shuttle effect” led by polysulfides dissolution, which is responsible to the severe active materials loss, low Coulombic efficiency, and anode corrosion.1−3 In recent years, numerous strategies focusing on novel electrolytes,4−7 separators,8−10 and particularly cathode materials11−13 have been attempted to tackle the “shuttle effect”. Among these efforts, cathode assemblies and modifications represent the most encouraging approach to inhibit polysulfides dissolution for mitigating the “shuttle effect”. Various micro/ nanostructured carbon materials, involving graphene oxides,14−18 carbon nanotubes,19−22 nanofibers,23−26 and hollow/porous carbon spheres,27−31 have been employed as conductive matrices to constrain sulfur within the carbon frameworks. Not only the conductivity of the cathode is enhanced but the dissolution of the polysulfides inside the carbon pores could be partially suppressed since the contact between polysulfides and the electrolyte are confined only at the orifices. However, polysulfides dissolution from the orifice openings would inevitably lead to gradual capacity decay. Successively, many third-party agents, including metal oxides and polymers, have been utilized to incorporate with C/S © 2015 American Chemical Society
Received: July 3, 2015 Revised: August 19, 2015 Published: August 19, 2015 20495
DOI: 10.1021/acs.jpcc.5b06373 J. Phys. Chem. C 2015, 119, 20495−20502
Article
The Journal of Physical Chemistry C
convergence,
