Colloidal dispersion of Nb2O5/reduced graphene oxide nanocomposites as functional coating layer for polysulfide shuttle suppression and lithium anode protection of Li-S battery
Graphical abstract
Colloidal dispersion of Nb2O5/reduced graphene oxide nanocomposites as functional coating layer for polysulfide shuttle suppression and lithium anode protection of Li-S battery.
Introduction
With the increasing demands of portable electrical devices and vehicle electrification, it is crucial to develop advanced electrochemical energy storage systems with high energy density [1], [2]. Among various rechargeable battery systems such as sodium sulfur batteries [3], [4], zinc-air batteries [5], [6], and lithium-air batteries [7], [8], lithium-sulfur batteries have been promising candidates due to their ultrahigh theoretical energy density of 2600 Wh kg−1 (three to five times of commercial lithium-ion batteries) [9], [10]. However, the practical application of lithium-sulfur (Li-S) batteries has been hindered by severe shuttle effect, originating from multi-step conversion of element sulfur. During the lithiation of sulfur, the solid cyclo-S8 transforms into high-order soluble polysulfides and ultimately to solid low-order polysulfides. Driven by the concentration gradient, the soluble intermediate high order polysulfides migrate from cathode side to anode side and react with lithium metal [11], [12], [13]. The diffusion of polysulfides are thought to be the main reason for low utilization of active sulfur and corrosion of lithium metal, leading to the severe capacity degradation and short cyclic life of the Li-S batteries.
To suppress the undesired shuttle effect of polysulfides, great endeavors have been devoted. The initial strategy was achieved by the encapsulation of sulfur in the nanopores of conductive carbons, such as CMK-3 mesoporous carbons [14], microporous carbon [15], [16], carbon nanotubes [17], [18] and graphene [19], [20], [21]. These porous structures can offer abundant pore volume capable of achieving adequate amount of sulfur loading, while high-surface-area frameworks can effectively hinder the polysulfides shuttling through physical adsorption [22]. However, such nonpolar carbon materials have weak adsorption for polar polysulfides, which alone cannot serve as the perfect host. Various types of polar functional groups on carbon-based materials have been demonstrated to increase the interaction between Li2Sn species and the electrode; these materials can generally be categorized into three types: heteroatom-doping (N [23], [24], [25], O [26], S [27]), metal oxides (TiO2 [28], MnO2 [29], La2O3 [30]), and transition-metal disulfides [31] (TiS2, ZrS2, VS2). Except the enhanced chemical adsorption, some metal oxides or sulfides such as Nb2O5 [32], MoS2 [33], CoS2 [34] have the ability of electrocatalyst, which could accelerate the polysulfide redox kinetics and thus to alleviate the polysulfide shuttling. Nevertheless, it is still a big challenge to rationally design a hybrid cathode system to reach a satisfactory polysulfides trapping, outstanding electronic conductivity and high sulfur loading. Moreover, these cathode materials intend to confine the soluble polysulfides within the electrodes, which cannot cope with already dissolved polysulfides.
Separator, as an essential component of electrochemical system, possesses interconnected channels with sub-micro diameters which plays the primary function in separating cathode and anode for preventing short circuit. This porous polymer membrane guarantees ion diffusion and electrolyte permeation but also provides pathway for polysulfides migration. Recently, the modification of separator with functional coating has proven to be effective in suppressing the migration of polysulfides across the separator [35], [36], [37]. This is typically realized by coating the separator with a electronegative polymeric layers, such as Nafion [35], lithium perfluorinated sulfonyl dicyanomethide [36], reduced graphene oxide/sodium lignosulfonate [37] and poly(acrylic acid) [38] to reject polysulfide anions via electrostatic repulsion. Conductive layer such as super P [39], activated carbon [40], carbon nanotube [41] or graphene films [42] were also used to block the migration of polysulfides as well as to serve as “upper current collector”. Being similar to the cathode design, polar metal oxide nanoparticles have been introduced into coating layer to enhance the chemisorption ability towards polysulfides via Lewis acid-base interactions [43]. Although these separators could effectively block the migration of polysulfides, it should be noted that the functional coating is mostly achieved by a slurry casting method. The thick coating layer in the system increases the weight of inactive component and the electrolyte uptake, which inevitably compromise the cell performance. Thus, the development of effective but lightweight coating layers is very important to the practical applications of the functional separator.
In this work, we demonstrate a multifunctional separator modified with an ultrathin conductive Nb2O5/reduced graphene oxide (rGO) nanocomposite (Nb2O5-rGO) layer for high effective blocking of the polysulfides shuttle and inhibiting the dendrite growth in the lithium metal anode (illustrated in Scheme 1). Nb2O5 is a unique electronic semi-conductor which has a very fast Li+ intercalation behavior in its bulk structure [44]. Our work revealed that Nb2O5 nanoparticles could deliver an electrocatalytic effect of sulfur redox, which dynamically promoted the kinetics of the polysulfides redox reaction, especially for the reduction of soluble Li2S6/Li2S4 to insoluble Li2S2/Li2S [32]. Herein, the ultra-thin Nb2O5 nanoparticles could be sufficiently and uniformly anchored in the rGO layer through a one-pot polyol synthesis. The present synthesis allows the obtained Nb2O5-rGO to form a highly dispersed colloidal solution, based on which the ultrathin coating layer is obtained through a simple vacuum-filtration without the addition of polymeric binder. The cooperative combination of high active Nb2O5 nanoparticles and conductive rGO layer should afford a good construction strategy to chemically and physically trap polysulfides within a functional coating layer. In addition, the presence of the Nb2O5 nanoparticles between condense coating oriented from the stack of rGO layer could create adequate transport channels for electrolyte and Li+ diffusion. Therefore, the modified separator enables high-sulfur-content cathode to achieve high specific capacity of 1328 mAh g−1 at 0.2C and 754 mAh g−1 retained after 200 cycles, without compromising the rate capability of Li-S batteries. Moreover, the presence of thin Nb2O5-rGO layer could also induce homogeneous Li+ deposition and prevent the lithium surface corrosion induced by polysulfide shuttle effect. These results indicate the modified separator is promising to optimize the electrochemical performance for Li-S batteries.
Section snippets
Preparation of Nb2O5-rGO colloidal solution
Graphene oxide (GO) is prepared from natural graphite flakes by a modified Hummers method. Typically, 0.3 g Niobium chloride (NbCl5) is dispersed in 30 mL ethylene glycol (EG) to get a homogeneous solution with magnetic stirring for 30 min. Then, NbCl5/EG solution and GO/EG colloidal solution (5 mg mL−1) are mixed under continuous magnetic stirring for another 2 h. Afterward, the obtained NbCl5-GO/EG suspension are added into a Teflon-lined stainless steel autoclave and heats at 180 °C for
Material characterization
The key to the modified separator is using highly dispersed Nb2O5 nanoparticle/reduced graphene oxide (Nb2O5-rGO) colloidal solution, based on which the flexible separators are obtained through a simple vacuum-filtration without the addition of any conductive agents or polymeric binder. The Nb2O5-rGO colloid solution is prepared by a facile solvothermal reaction of NbCl5 and GO in ethylene glycol (EG) solution, as illustrated in Fig. 1a. During the solvent thermal reaction, GO is reduced to rGO
Conclusions
In summary, Nb2O5-rGO colloidal dispersion have been prepared by solvothermal reaction and coated on commercial PP separator via facile vacuum filtration to apply as modified separators for Li-S batteries. With the assistant of Nb2O5-rGO functional layer, the resulting sulfur cathode exhibits excellent cycling stability and high Coulombic efficiency as well as good rate capability. The sulfur cathode shows higher specific capacity of 1328 mAh g−1 at 0.2C and 754 mAh g−1 retained after 200
CRediT authorship contribution statement
Qingyan Ma: Conceptualization, Writing - original draft. Mengfei Hu: Formal analysis, Methodology. Yuan Yuan: Software. Yankai Pan: Visualization, Validation. Mingqi Chen: Software, Validation. Yayun Zhang: Formal analysis, Supervision. Donghui Long: Writing - review & editing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was partly supported by National Natural Science Foundation of China (No. 21576090), and Fundamental Research Funds for the Central Universities (222201718002).
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