Letter pubs.acs.org/NanoLett
Improving Lithium−Sulfur Battery Performance under Lean Electrolyte through Nanoscale Confinement in Soft Swellable Gels Junzheng Chen,† Wesley A. Henderson,† Huilin Pan,† Brian R. Perdue,‡ Ruiguo Cao,† Jian Zhi Hu,† Chuan Wan,† Kee Sung Han,† Karl T. Mueller,† Ji-Guang Zhang,† Yuyan Shao,*,† and Jun Liu*,† †
Joint Center for Energy Storage Research (JCESR), Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡ Joint Center for Energy Storage Research (JCESR), Sandia National Laboratories, Albuquerque, New Mexico 87185, United States S Supporting Information *
ABSTRACT: Li−S batteries have been extensively studied using rigid carbon as the host for sulfur encapsulation, but improving the properties with a reduced electrolyte amount remains a significant challenge. This is critical for achieving high energy density. Here, we developed a soft PEO10LiTFSI polymer swellable gel as a nanoscale reservoir to trap the polysulfides under lean electrolyte conditions. The PEO10LiTFSI gel immobilizes the electrolyte and confines polysulfides within the ion conducting phase. The Li−S cell with a much lower electrolyte to sulfur ratio (E/S) of 4 gE/gS (3.3 mLE/gS) could deliver a capacity of 1200 mA h/g, 4.6 mA h/cm2, and good cycle life. The accumulation of polysulfide reduction products, such as Li2S, on the cathode, is identified as the potential mechanism for capacity fading under lean electrolyte conditions. KEYWORDS: Li−S battery, lean electrolyte, high energy cell, gel capture, failure mechanism
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(lean electrolyte condition).18,21 It is increasingly recognized now that good electrochemical performance obtained with flooded electrolytes cannot be reproduced under lean electrolyte conditions.22−25 Quan et al. reported an E/S ratio at 3.5 mLE/gS operation under high sulfur loading, but the cell only lasts for 10 cycles. The change to lean electrolyte condition is critical for high pack-level energy density but places severe restrictions on wetting, charge transport between the interface, and electrochemical reaction kinetics. In addition, when a Li−S cell operates under a lean electrolyte condition, its charge/ discharge behavior, cell failure model, and so forth may be substantially different from those with flood electrolyte (i.e., with large E/S ratio) as we have always been doing. Certainly, to gain good fundamental understanding, one has to be able to run a Li−S cell for extended cycles under lean electrolyte conditions. In this paper, we demonstrate a soft PEO10LiTFSI polymer gel as a nanoscale reservoir to trap the polysulfides under lean electrolyte conditions.26−28 In this approach, the electrolyte and the polysulfides are confined in the swellable, flexible polymer phase which by itself is also ion conducting, thus provides more efficient encapsulation and better ion transport properties. Poly(ethylene oxide) (PEO) has been widely considered as a polymer electrolyte (mixed with lithium salts) for batteries
i−S batteries have been widely studied as next-generation energy storage technology since sulfur has a high theoretical capacity (1672 mA h/g), low cost, and high earth abundance.1,2 However, before the broad commercialization of Li−S batteries, several technical challenges have to be addressed, including Li metal anode degradation,3,4 polysulfide dissolution,5,6 and electrolyte decomposition.7−9 The Li−S reaction involves the dissolution of reaction intermediate lithium polysulfides (Li2Sn) in the electrolyte which promotes the reaction kinetics; however, the dissolution of Li2Sn also causes the loss of active material from cathode architecture, as well as side reactions between Li2Sn and Li metal (and possible side reaction with electrolytes).10−12 Great progress has been made using rigid nanostructured porous carbon or metal oxide hosts to encapsulate the sulfur on the cathode, together with advances in electrolyte additives and other electrode protection strategies.13−16 The rigid encapsulation approach relies on the interfacial binding between the substrate and the polysulfides and charge transport across the interfaces. The majority of investigation reported in the literature was conducted with a high electrolyte amount. The careful analysis suggested that the amount of electrolyte needs to be significantly reduced in order for Li−S battery technology to achieve the desired high energy density.17,18 Currently, most studies use a high electrolyte amount with an electrolyte to sulfur (E/S) ratio in the range of 10−50 mLE/gS (flooded electrolyte condition).19,20 There have been very few reports on rechargeable Li−S batteries using a low electrolyte amount with an E/S ratio less than 5 mLE/gS © 2017 American Chemical Society
Received: January 30, 2017 Revised: April 10, 2017 Published: April 27, 2017 3061
DOI: 10.1021/acs.nanolett.7b00417 Nano Lett. 2017, 17, 3061−3067
Letter
Nano Letters
Figure 1. (a) Up: solid binder powder before adding the electrolyte, bottom: swelling and electrolyte uptake test of binders in the 1 M LiTFIS DME/DOL electrolyte. 1-PEO10LiTFSI, 2-LA133, 3-CMC, 4-PVDF. PEO10LiTFSI swelled more than 4 times in volume and uptaked ∼10 mL electrolyte/g PEO10LiTFSI. (b) Solvent evaporation test of the electrolyte infiltrated in different binders showing a strong absorption ability of PEO based gel capture mediate; (c) XRD patterns of PEO10LiTFSI composite before and after adding LiTFSI indicating an amorphous region due to the interaction of Li+ with the ether group; (d) UV−vis spectra of 18 mM Li2S8 swelled in the PEO electrolyte and the polysulfide concentration changes from 18 mM to ∼2 mM after resting for 12 h; (e) 13C solid-state NMR spectra of PEO based composite before and after adding LiTFSI and Li2S8, showing CH2 chain peak narrowing, indicating an increased carbon main chain flexibility due to the interaction of Li+ with ether group; (f) schematic diagram of the electrode with PEO10LiTFSI nanofilm coating and the confinement of polysulfides by the swelled PEO10LiTFSI gel.
because it has a strong Li+ solvating ability.29−31 Several groups also reported PEO as a binder material for Li−S systems.32−35 However, pure PEO tends to crystallize after solvent evaporation, limiting its binding ability and practical application in sulfur cathodes.36 Here, we use a combination of PEO and LiTFSI to form a swelled amorphous gel-like polymer nanocoating film on conductive carbon. Compared to rigid high surface area carbon, this preformed PEO-LiTFSI gel functions as a soft media for Li-ion conducting, electrolyte wetting, and serves as a swellable reservior for retaining the electrolyte and polysulfides near the conducting carbon surfaces, thus enable extended cycling of lean electrolyte Li− S cells. This study also provides a new insight of potential degradation mechanism under lean electrolyte conditions that are different from those under flooded electrolyte conditions. Demonstration of Nanoscale Confinement with Soft Swellable Gels for the Lean Electrolyte Li−S Operation. First, we demonstrate that the PEO10LiTFSI soft gel as swellable reservoir has excellent capability to solvate and retain the electrolyte in the mixture. Figure 1a shows the swelling and electrolyte uptake tests of different binders with the commonly used 1 M LiTFSI/DME-DOL with the E/S ratio = 4 gE/gS (3.3 mLE/gS) The PEO10LiTFSI is well-swelled in the electrolytes and absorbs 10 mL/g electrolyte within the gel layer phase. There is no obvious interaction with other binders: poly(vinylidene fluoride) (PVDF), sodium carboxymethyl cellulose (CMC), and LA133. The electrolyte retention is further demonstrated by evaporation ratio test. The evaporation of adsorbed electrolyte is shown in Figure 1b. Due to its low boiling point, the electrolyte in traditional binders quickly evaporates. However, the electrolyte evaporation loss in PEO10LITFSI is significantly lower (
