Optimization of Pore Structure of Cathodic Carbon Supports for

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Optimization of Pore Structure of Cathodic Carbon Supports for Solvate Ionic Liquid Electrolytes-Based Lithium-Sulfur Batteries Shiguo Zhang, Ai Ikoma, Zhe Li, Kazuhide Ueno, Xiaofeng Ma, Kaoru Dokko, and Masayoshi Watanabe ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09989 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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ACS Applied Materials & Interfaces

Optimization of Pore Structure of Cathodic Carbon Supports for Solvate Ionic Liquid Electrolytes-Based Lithium-Sulfur Batteries Shiguo Zhang, ‡ Ai Ikoma,‡ Zhe Li, Kazuhide Ueno, Xiaofeng Ma, Kaoru Dokko, and Masayoshi Watanabe* Department of Chemistry and Biotechnology, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan. KEYWORDS. Porous carbon, lithium-sulfur battery, electrolyte, solvate ionic liquids, polysulfide-insoluble, pore volume.

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ABSTRACT

Lithium-sulfur (Li-S) batteries are a promising energy-storage technology owing to their high theoretical capacity and energy density. However, their practical application remains challenges because of the serve shuttle effect caused by the dissolution of polysulfides in common organic electrolytes. Polysulfide-insoluble electrolytes, such as solvate ionic liquids (ILs), have recently emerged as alternative candidates and showed great potential in suppressing the shuttle effect and improving the cycle stability of Li–S batteries. Redox electrochemical reactions in polysulfide-insoluble electrolytes occur via a solid-state process at the interphase between the electrolyte and the composite cathode; therefore, creating an appropriate interface between sulfur and a carbon support is of great importance. Nevertheless, the porous carbon supports established for conventional organic electrolytes may not be suitable for polysulfide-insoluble electrolytes. In this work, we investigated the effect of the porous structure of carbon materials on the Li-S battery performance in polysulfide-insoluble electrolytes using solvate ILs as a model electrolyte. We determined that the pore volume (rather than the surface area) exerts a major influence on the discharge capacity of S composite cathodes. In particular, inverse opal carbons with three-dimensionally ordered interconnected macropores and a large pore volume deliver the highest discharge capacity. The battery performance in both polysulfide-soluble electrolytes and solvate ILs was used to study the effect of electrolytes. We propose a plausible mechanism to explain the different porous structure requirements in polysulfide-soluble and polysulfide-insoluble electrolytes.


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1. INTRODUCTION Lithium−sulfur (Li−S) batteries are considered as one of the most promising “beyond Li-ion” technologies because of their high theoretical capacity (1672 mAh/g) and high theoretical energy density (2600 Wh/kg or 2800 Wh/L).1-8 Li-S batteries are also attractive because sulfur is naturally abundant, nontoxic, and inexpensive. Although significant progress has been made recently,9 the practical applications of Li–S batteries have not yet been realized because of their poor electrochemical performance, limited by several issues.10 First, both sulfur and the discharge product Li2S are electrically insulating, making it difficult to achieve the theoretical speciﬁc capacities and projected energy densities. In addition, the active materials undergo severe volumetric expansion/contraction between sulfur and Li2S during repeated cycles, which results in decreased mechanical integrity and poor electrode stability.11,12 Moreover, the lithium polysulfide intermediates (Li2Sm, especially 4≤m≤8) tend to dissolve in most organic electrolytes, are transported through the electrolyte, and dissipate energy at the lithium anode due to the parasitic chemical reactions between Li2Sm and the Li metal anode. The undesired shuttle effect results in the gradual loss of active material available at the cathode after each cycle. Consequently, Li–S batteries suffer from a low practical capacity, low Columbic efficiency, and poor cycling stability. Previous studies have focused on the construction of nanostructured composite cathodes. The composite cathodes are typically composed of sulfur, a conductive material, and a polymer binder. The conductive materials are used as sulfur supports and form an electronic conduction path within the composite electrode. Nanostructured supporting materials, such as porous carbons, graphene, graphene oxide, and conductive polymers, have been intensively investigated.9,13-17 Porous supports provide the insulating active material with a stable electrical contact, facilitate electrolyte transport and electron transfer across the composite


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cathode, help to accommodate the volume change, and kinetically alleviate the shuttle effect by retaining Li2Sm in the pores.18 Nevertheless, this approach cannot thermodynamically prevent the shuttle effect because Li2Sm can still dissolve in the electrolyte after several discharge cycles due to the direct contact between the electrolyte and the embedded sulfur.

Figure 1. Two typical Li–S cells based on (a) polysulfide-soluble and (b) polysulfide-insoluble electrolytes. Reproduced with permission.19 Copyright 2015, Wiley. In addition to exploiting the sulfur cathode, the development of novel electrolytes for Li–S batteries to thermodynamically suppress the dissolution of Li2Sm and prevent the shuttle effect is of great importance.19-25 The most commonly used organic electrolytes of Li-S batteries are mixed ether solvents, such as dimethoxyethane (DME) and 1,3-dioxolane (DOL) (1:1 v/v) containing

1

M

of

a

dissociative

lithium

salt

(typically

lithium

bis(trifluoromethanesulfonyl)amide (LiTFSA)).26 The ether electrolytes contain excess free solvent molecules, and the ether oxygen atoms preferentially coordinate to Lewis acidic Li+ ions


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(owing to their strong donor ability of the lone pair electrons). This provides a high ability to dissociate alkali metal salts and thus a high solubility of the polysulfide species, especially for high-order lithium polysulfides (Li2Sm, 4≤m≤8). For example, S solubility (as Li2Sm) can exceed 10 M in tetrahydrofuran.27 The solubility of one of the most soluble polysulfides with the nominal formula Li2S8 can be up to ≈ 6 M (total atomic S concentration) in a 1 M LiTFSA/G4 electrolyte (G4: tetraethylene glycol dimethyl ether or so called tetraglyme).28 Therefore, the polysulfide-soluble electrolytes function as a catholyte in Li–S cells after the first discharge, as shown in Figure 1a. In this case, redox reactions of S species take place through a solid–liquid process between the electronic conductor (C/S composite) and Li+ conductor (electrolyte containing polysulfides). Even though the dissolution of S species enables fast reduction kinetics, the shuttle-effect results in a loss of active materials from the cathode. An efficient method to alleviate the shuttle effect of the ether electrolytes is by using small amount of lithium nitrate (LiNO3) as an additive, which can help to form a stable solid–electrolyte interphase on the Li metal anode.29 However, the discharge cutoff voltage for the additive-containing electrolytes must be increased to a much safer potential (usually above 1.7 V) to avoid the irreversible reduction of LiNO3 on the cathode;30 this narrow voltage window inevitably causes a loss of capacity.31 In addition, the continuous consumption of LiNO3 on the Li metal anode limits its application for long-term cycling.


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Figure 2. Structures of the solvate IL, [Li(G4)][TFSA], and co-solvent, HFE. Table 1. Characteristic properties of ether electrolyte and solvate ionic liquids.a σ

η

d

Li2S8b

/mS cm−1

/mPa s

/g cm−3

/mM

0.51c

1.6

94.6

1.40

7.69d

[Li(G4)1][TFSA]/4HFE 1.04

0.51

5.2

5.22

1.49

1.25d

DOL/DMEe

0.49

13.4

1.24

1.10

> 755f

Electrolytes

C(Li+)

t+

/M dm−3

[Li(G4)1][TFSA]

a

2.75

1.00

C(Li+), t+, σ, η, and d are molar concentration of lithium ion, Li+ transfer number, conductivity,

viscosity, and density, respectively. b Solubility of Li2S8. c Data from ref. 32. d Data from ref. 33.

e

Measured in this work. f Referring to data obtained from ethers.27,28,34 Several new electrolytes (e.g., ionic liquids (ILs)23,35-37 and concentrated solutions25,38), which are distinct from the catholyte-type electrolytes, showed great potential in suppressing the shuttle effect and improving the cycle stability. In particular, certain glyme-Li salt molten complexes (e.g., [Li(G4)][TFSA], Figure 2), as a new family of solvate ILs,39 have proven to be promising electrolytes for Li-S batteries.19,33,40 The solvate ILs can be obtained by simply mixing equimolar amounts of glymes and Li salts, whereas conventional IL electrolytes require complicated synthesis and additional lithium salt. Solvate ILs are intrinsically Li+ conductive, have a high Li+ concentration, and a high Li+ transference number.19,32,33,40-45 More importantly, all of the oxygen atoms of the ether molecules in the solvate ILs coordinate with the doped Li salts. Owing to the weak Lewis basicity of [TFSA]− anions, their solvation power is too weak to


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solvate the Li+ ions of Li2Sm.41 As a result, the solubility of Li2Sm is greatly depressed in the solvate ILs.33,40 For example, Li2S8, which is highly soluble in conventional organic electrolytes,27 showed a very low solubility of

Optimization of Pore Structure of Cathodic Carbon Supports for

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