Poly(3,4-ethylene-dioxythiophene)-poly(styrenesulfonate) glued and graphene encapsulated sulfur-carbon film for high-performance free-standing lithium-sulfur batteries
Graphical abstract
PEDOT-PSS glued and graphene encapsulated hollow sulfur-carbon film was prepared.
Introduction
The development of next-generation batteries with energy density higher than lithium ion batteries is highly sought to make the next leap in the energy storage research. This is because there is an urgent need in replacing our dependence on fossil fuel use and also in contributing to the development of environmentally friendly technologies. These new batteries will find uses in the vehicle electrification and in the grid energy storage to name a few examples [1], [2], [3]. Among the different types of promising batteries, lithium-sulfur (Li-S) battery holds great potential because of its high energy density of 2600 Wh kg−1 and high specific capacity of 1673 mAh g−1, which are much higher than the state-of-the-art LiNi1/3Mn1/3Co1/3O2-graphite system (1400 Wh kg−1 and 274 mAh g−1, respectively) [4], [5], [6]. Meanwhile, elemental sulfur is also abundant, nontoxic and inexpensive. Hence, Li-S batteries are of particular promise for the next-generation energy storage devices. Despite these advantages, several bottlenecks still need to be addressed in Li-S battery before it is used in practical applications. The shuttle effect, which is caused by the back and forth movement of soluble polysulfide intermediates, typically leads to a rapid capacity decay and low Coulombic efficiency [7]. Moreover, the large volumetric changes during the charging-discharging process and the sluggish kinetics of bulk sulfur typically result in poor electrochemical performance. To address these drawbacks, efforts have been devoted to designing novel structured sulfur-based electrodes as cathode materials. The use of carbon-based hosts such as graphene [8], [9], [10], [11], [12], carbon nanotubes [13], [14] or mesoporous carbon [15], [16] have been shown to trap the soluble polysulfides by physical restraint or by chemical bonding, thus alleviating the dissolution of sulfur during the charging/discharging process [17]. However, the weak adhesion of sulfur to the carbon and the heterogeneity of the sulfur in the carbon have been shown inefficient in preventing the detachment of lithium sulfides from the carbon surface and the self-aggregation of polysulfides [18], [19], [20]. Polyvinylpyrrolidone (PVP) has been shown to have a strong affinity to lithium polysulfides. For example, by encapsulating the sulfur with PVP, the polysulfide dissolution was minimized and the detrimental effects of the volume expansion of sulfur was mitigated, which lead to an overall improved cycling stability [21]. However, the conductivity of the electrodes still remained low and an enhancement in the overall conductivity is anticipated to result in an enhanced electrochemical performance. Furthermore, aluminum foil is the present standard current collector material in these electrodes. This material typically accounts for more than 30% of the total electrode weight [22], [23]. It is also often observed that active materials delaminate from the current collector during the charging/discharging process due to the inherently weak contact and mechanical property mismatch between the metal current collector and the volume expansion of the active materials during cycling [24].
Recent works on three-dimensional (3D) metal foam [25] and carbon foam [22] have used these materials as alternative substrates to achieve highly flexible electrodes because of their remarkable mechanical properties and intrinsic high surface area that is critical for achieving high energy density [26], [27]. Among these materials, graphene is proving to be one of the most promising flexible substrates for achieving free-standing electrodes due to its superior conductivity, high mechanical flexibility and low density [28]. Moreover, graphene with high conductivity and wrinkled structure have been investigated as current collectors to replace the metal current collectors and to improve the adhesion with active materials [29]. However, few attempts have been made to utilize the merits of both the conductive polymer and the carbon-based materials to further enhance the performance of the free-standing Li-S batteries. In this work, we rationally designed a free-standing sulfur electrode consisting of PVP that are homogeneously confined within the conductive matrix of graphene and poly(3,4-ethylene-dioxythiophene)-poly(styrene sulfonate) (PEDOT-PSS), referred to as PVPS/PED@RGO. We here show that each of the electrode component we used have unique function. For example, the hollow PVPS with core-shell structure minimizes the polysulfide dissolution and accommodates the volume expansion of sulfur during cycling. Moreover, the graphene and conductive polymer matrix simultaneously function as the current collector and as the conductive binder for fast charge transport and for maintaining the integrity of the electrode during cycling [30].
Section snippets
Preparation of graphene oxide (GO)
GO was prepared from natural graphite powder by a modified Hummers' method [31]. First, 5 g of graphite powder and 2.5 g of sodium nitrate (NaNO3) were added into 115 mL of concentrated H2SO4 in an ice bath. Then 15 g KMnO4 was slowly added in the solution under stirring, and then mixed with 230 ml deionized water and kept at 95 °C for 1.5 h under stirring. Then 10 mL 30% H2O2 was added into the solution until the color of the mixture turned yellow. The GO dispersion was then subjected to
Results and discussions
Fig. 1a shows the schematic illustration of how the free-standing PVPS/PED@RGO composite films were prepared. Briefly, PVPS, PEDOT-PSS and vitamin C were mixed with GO solution (refer to experimental section for full details). It was found that PVPS dispersed very well in the solution in the presence of GO and PEDOT:PSS because both materials served as the dispersing agent. A mechanically robust aerogel was obtained following a hydrothermal treatment of the mixture. The presence of vitamin C in
Conclusions
We have successfully synthesized free-standing PVPS/PED@RGO electrodes. The excellent electrochemical performance of these electrodes can be ascribed to the synergistic effect of core-shell hollow sulfur-PVP particles, the wrapping of graphene sheets with rippled structure, and the enhancement of conductivity through the addition of PEDOT-PSS. We believe that such a simple and scalable approach may provide practical avenues for the fabrication of various graphene-based electrodes with enhanced
Acknowledgement
This work was supported by the Applied Fundamental Foundation of Sichuan Province (2014JY0202), the R&D Foundation of China Academy of Engineering Physics (2014B0302036), the Science Foundation for Distinguished Young Scholars of Sichuan Province (2016JQ0025 and 2017JQ0036) and National Natural Science Foundation of China (No. 21401177, 51403193 and 21501160), the “1000plan” from the Chinese Government, and the Collaborative Innovation Foundation of SiChuan University (XTCS2014009). J.M.R.
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2019, Progress in Polymer ScienceCitation Excerpt :PSS was usually used to dope PEDOT to improve the conductivity [176–178]. Charged PEDOT:PSS with polysulfide trapping ability became the superior sulfur coating materials in Li-S batteries [167,179,180]. Recently, the compact, flexible and free-standing films with a sandwich structure were designed simply by vacuum filtration (Fig. 19A), in which nanosulfur was homogeneously coated by graphene and PEDOT:PSS (Fig. 19B) [181].