Film Properties of Electropolymerized Polypyrrole for a Sulfur/Ketjenblack Cathode in Lithium Secondary Batteries

Abstract

A novel polypyrrole (PPy) film was investigated to determine the optimal conditions for operation in a Li/S battery. The PPy film was prepared by oxidative electropolymerization to improve the Li/S battery performance, as reported in our previous paper. In such a system, the PPy film was coated directly on the S/Ketjenblack cathode to solve the problem of polysulfide dissolution. The optimum PPy film preparation conditions to prevent polysulfide dissolution and to promote Li + permeability were determined by varying the PPy polymerization bath composition and polymerization potential. As a result, the inclusion of 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in the polymerization bath (0.1 M pyrrole in 1-methyl-1-butylpyrrolidinium bis(trifluoromethanesulfonyl)imide) was found to be the most important factor for producing a PPy film with a high Li + transport number (t Li + ≈ 1). A polymerization potential of 4.5 V versus Li/Li + was shown to be optimum for the promotion of Li + permeability. The mechanism by which the PPy film prevents polysulfide dissolution and increases Li + permeability is discussed by analyzing the SEM, CV, XPS, and 13 C solid-state NMR data. Lithium/sulfur (Li/S) batteries have attracted substantial attention as a next-generation battery candidate because their theoretical energy density (∼2500 W h kg −1 ) is much higher than the current C/LiCoO 2 system (387 W h kg −1 ). [1][2][3][4] In addition, sulfur exhibits several impressive characteristics, such as abundance in nature and low toxicity. However, Li/S batteries have several intrinsic problems that affect their practical application. Some of the issues associated with the sulfur cathodes include the low ionic and electronic conductivities of sulfur and Li 2 S, large volume expansion of sulfur upon lithiation (∼80% expansion), and dissolution of intermediate products, namely lithium polysulfides (Li 2 S x , where 4 ≤ x ≤ 8), into the electrolyte. 6,7 This dissolution of polysulfide causes "redox shuttle phenomena," which leads to overcharging (low coulombic efficiency) and rapid capacity fading. 8 Various strategies have been developed to solve these problems. 9Since Nazar's group overcame the low conductivity problem of the sulfur cathode and achieved cycle stability by producing a sulfurordered mesoporous carbon (CMK-3) composite, 10 various carbon materials have been studied as hosts for sulfur. 11-15In order to suppress polysulfide dissolution, ionic-liquidbased electrolytes have been investigated. 16,17 Ionic-liquid-based electrolytes show excellent polysulfide dissolution inhibition, while batteries containing the ionic liquid-based electrolytes show relatively high resistances, owing to the high viscosities of these electrolytes. However, the high cost of ionic liquids hinders their practical application compared with commonly used organic electrolytes. There have been many reports on methods for suppressing dissolution of polysulfides by coating or modifying the surface of an active material with a polymer using c...

Facile Synthesis of Sulfur–Polypyrrole as Cathodes for Lithium–Sulfur Batteries

“…Figure c and d presents discharge and charge curves of sulfur and S‐PPy after different cycles, in which the current density and potential range were set at 200 mA g −1 and 1.5–3.0 V, respectively. Both sulfur and S‐PPy shown in Figure c and d exhibit two typical discharge plateaus at about 2.0 and 2.3 V, which are ascribed to the multiple reaction mechanism of elemental sulfur and lithium metal, as described elsewhere, and the inferences are consistent with the CV measurements. The upper and the lower plateaus correspond to changes from cyclic S 8 to higher‐order lithium polysulfides (Li 2 S n , 4< n <8) and to lower‐order polysulfides ( n <4), down to Li 2 S 2 or Li 2 S, respectively.…”

Evaluating Sulfur-Composite Cathode Material with Lithiated Graphite Anode in Coin Cell and Pouch Cell Configuration

“…Charge/discharge profiles and cycling performance of the halfcell coin cell are presented in Figures 3A,B, respectively. The initial discharge capacity of the cell at 0.2 C was about 1700 mAh g −1 , which after an intrinsic initial irreversible capacity drop stabilized around 1,300 mAh g −1 for 100 cycles (Hara et al, 2015;Nakamura et al, 2016;Wang et al, 2018). The low potential discharge plateau at the first cycle (ΔV ∼ −0.35 V) and large initial discharge capacity is typical behavior for sulfurized polyacrylonitrile composite materials (Wang et al, 2018).…”

“…Several sulfur/conductive polymer/conductive carbon ternary composites have been developed and offer an inexpensive alternative, yet their properties can be further optimized (Liang et al, 2016). For instance, polypyrrole-coated sulfur/ ketjen black (S/KB) with 1,047 mAh g −1 initial discharge capacity decays in the performance during cycling due to the dissolution of polysulfides (Nakamura et al, 2016). The composites of sulfur with conductive polymer-polyacrylonitrile have been intensely investigated because it exhibited a capability to accommodate the sulfur volume change upon cycling, prevent polysulfide dissolution, and act as conductivity enhancer (Wang et al, 2002;Peng et al, 2017).…”

The use of polymers in Li‐S batteries: A review

“…The use of conductive polymers as an interlayer has also received attention with examples of electrically conductive polymers such of PANI, PPy, and PEDOT recently demonstrated in addition to ionically conductive Nafion based coatings . Kim et al reported on the utilization of Nafion coated cathodes with a particular emphasis on the importance of complete cathode coverage for maximizing improved electrochemical performance .…”