Journal of Power Sources xxx (xxxx) xxx–xxx

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Journal of Power Sources

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Lithiated Nafion as polymer electrolyte for solid-state lithium sulfur batteries using carbon-sulfur composite cathode

∗ Jing Gaoa,b, Chunshui Suna,b, Lei Xua, Jian Chena, , Chong Wanga, Decai Guoa, Hao Chena a Advanced Rechargeable Battery Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China b University of Chinese Academy of Sciences, Beijing 100049, China

HIGHLIGHTS GRAPHICAL ABSTRACT

• Lithiated Nafion (Li-Nafion) is applied for solid-state Li-S batteries. • Li-Nafion resin plays roles of solid- state electrolyte and binder in cathode. • A layer of Li-Nafion resin is fabricated between cathode and Li-Nafion mem- brane. • A Li-S battery with the interlayer and resin binder exhibits better perfor- mance. • Lithium dendrites are suppressed by single-ion conducting Li-Nafion mem- brane.

ARTICLE INFO ABSTRACT

Keywords: Due to flexible property and light weight, the lithiated Nafion membrane swollen with PC (PC-Li-Nafion) has Solid-state lithium sulfur cells been employed as both solid-state electrolyte and separator to fabricate solid-state Li-S cells. The electrochemical Single-ion conductor measurements of PC-Li-Nafion membrane show that its Li-ion transference number is 0.928, ionic conductivity fi − − Lithiated Na on of 2.1 × 10 4 Scm 1 can be achieved at 70 °C and its electrochemical window is 0 ∼ +4.1 V vs. Li+/Li. It is Carbon-sulfur composites observed that the Li dendrites are suppressed by using PC-Li-Nafion membrane due to its single-ion conducting Lithium anode property. The amounts of Li-Nafion resin binder and conductive carbon in the cathode are optimized as 40% and Interlayer 10% respectively to make a balance of ionic and electronic conductivities. A thin-layer Li-Nafion resin with a thickness of around 2 μm is fabricated between the cathode and PC-Li-Nafion membrane to improve the inter- facial contact and further enhance the specific capacity of the cell. When measured at 70 °C, the Li-S cell delivers − − a reversible specific capacity of 1072.8 mAh g 1 (S) at 0.05 C and 895 mAh g 1 (S) at 1 C. The capacity retention at 1 C is 89% after 100 cycles. These results suggest that high-performance solid-state Li-S cells can be fabricated with the Li-Nafion polymer electrolyte.

1. Introduction state active materials [1]. In addition, sulfur and Li resources are abundant and cheap. However, the practical application of Li-S bat- With an anode of lithium (Li) and a cathode of sulfur, lithium-sulfur teries is mainly hindered by its poor cyclability which can be caused by (Li-S) batteries possess the highest theoretical energy density of the “shuttle effect” arising from the lithium polysulfides generated −1 2600 Wh·kg among the rechargeable batteries composed by solid- during the conversion of S to soluble Li2Sn (4 ≤ n ≤ 8), or the side

∗ Corresponding author. E-mail address: [email protected] (J. Chen). https://doi.org/10.1016/j.jpowsour.2018.01.063 Received 27 October 2017; Received in revised form 8 January 2018; Accepted 21 January 2018 0378-7753/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Gao, J., Journal of Power Sources (2018), https://doi.org/10.1016/j.jpowsour.2018.01.063 J. Gao et al. Journal of Power Sources xxx (xxxx) xxx–xxx reactions between the Li anode and the organic solvents, or the reac- temperature [24], which is larger than that of many single-ion con- tions between the Li and lithium polysulfides crossing over the se- ducting polymer electrolytes reported previously [18]. Li-Nafion parator, etc. [2]. Besides, by using 1,3-dioxolane (DOL) and 1,2-etha- membrane has been employed as both electrolyte and separator for the nediol dimethyl ether (DME) as the solvent of electrolyte in current Li-S fabrication of Li-ion cells. Liu et al. used Li-Nafion membrane swollen batteries, the risks of safety are increased due to the low flash point and with ethylene carbonate and propylene carbonate to fabricate a cell of 2+ boiling point. To replace the organic liquid electrolyte by the solid-state LiMn2O4/Li-Nafion membrane/Li to prevent the dissolution of Mn electrolyte is expected to decrease the safety issues and improve the from the cathode due to the absence of HF derived from LiPF6 salt in the cyclability of Li-S batteries [3,4]. organic liquid electrolyte [25]. Liu et al. fabricated a solid-state Li-ion However, there are several challenges in the research and devel- cell by using the Li-Nafion membrane swollen with organic solvent as opment of solid-state Li-S batteries. Oxide solid electrolytes (OSEs) and electrolyte and separator, LiFePO4 as cathode and Li as anode. The cell − sulfide solid electrolytes (SSEs), which are two kinds of inorganic solid- exhibited a reversible specific capacity of 126.6 mAh·g 1 at 60 °C [26]. state electrolytes, exhibit the largest ionic conductivities of Recently, Bauer et al. confirmed that a thin-layer Li-Nafion resin coated − − − − ∼10 3 Scm 1 and ∼10 2 Scm 1 at room temperature, respectively on polypropylene membrane can suppress the diffusion of lithium [5]. However, OSEs are non-flexible, resulting in high interface re- polysulfides in the Li-S cell with ether electrolyte solutions [27,28]. sistance with the sulfur-based cathodes [6]. Thus, when OSEs are em- Besides, Song et al. coated a several-micron-thick-layer H-Nafion resin ployed to fabricate a Li-S battery, organic liquid electrolyte is always on the surface of Li metal anode to suppress the inhomogeneous elec- applied as the co-electrolyte to enhance the active interface between trodeposition of Li [29]. However, to the best of our knowledge, there is “solid-state electrolyte/active materials” and “solid-state electrolyte/ no report on solid-state Li-S cells with Li-Nafion membrane as solid- cathode” [7,8]. SSEs are sensitive to moisture and usually non-stable state electrolyte and separator without lithium salts and ether organic when contacting with Li metal [9]. Due to the intrinsic electronic and solvent. ionic insulation of sulfur and lithium sulfide, the cathodic reaction in a In this paper, the solid-state Li-S cells have been fabricated with Li- solid-state Li-S battery only occurs at the “tri-phase boundary” of “S Nafion membrane as both electrolyte and separator, carbon-sulfur (C-S)

(Li2S)/conductor/solid-state electrolyte” [10]. Moreover, the sulfur and composites as cathode active material, and Li metal as anode. This work solid-state electrolyte in the cathode are usually separated due to the has investigated the structure and electrochemical properties of Li- large volume expansion (can be larger than 70%) and shrink during Nafion membrane, the morphology, surface area and pore structure of discharging and charging in Li-S batteries [1,11]. Besides, the pre- the prepared carbon and C-S composites. In addition, the electro- paration of large-area thin-layer solid-state electrolyte membrane with chemical stripping-plating behaviors of Li metal with Li-Nafion mem- high ionic conductivity remains a challenge [9]. In addition, the density brane have also been explored. The electrochemical performance of the − of solid-state electrolyte, such as 4–5gcm 3 for OSEs and around solid-state Li-S cell is improved by recasting a thin-layer Li-Nafion resin − − 2–3gcm 3 for SSEs, is usually larger than the value of 1 g cm 3 for on the surface of cathode and optimizing the amount of Li-Nafion resin organic liquid electrolyte, which will lower the energy density of solid- in the cathode. The as-prepared Li-S cell shows excellent electro- state Li-S batteries [3]. chemical performance. Solid polymer electrolytes (SPEs) exhibit lower ionic conductivity of − − 10 6 Scm 1 at room temperature; however, their flexible property 2. Experimental makes them easy to produce a large-area membrane, facilitates various designs for batteries and alleviates the impact of volume changes of 2.1. Materials electrodes. In addition, SPEs have relatively light weight due to their − low density of ∼1.2 g cm 3, low interface resistance and high com- Nafion 211 membrane in H+ form (H-Nafion membrane, thickness: patibility with both anode and cathode [12–14]. At present, SPEs em- 25 μm) was purchased from Dupont Company, the chemical structure of ployed in solid-state Li-S batteries are usually fabricated by dissolving which is illustrated in Fig. S1a. Propylene carbonate (PC) and LiClO4 alkali metal salts in the polymer matrix, such as polyethylene oxide- were purchased from Zhangjiagang Guotai Huarong Co., China. lithium bis-(trifluoromethanesulfonyl)imide (PEO-LiTFSI) [15]. Cations LiOH·H2O (99%, aladdin), N-methyl pyrrolidone (NMP) (UP, acros), and anions are mobile in these polymer electrolytes, which makes their polyethylene oxide (PEO, aladdin, MW = 600,000) were purchased and Li-ion transference numbers usually lower than 0.5 [16,17]. In other used without further purification. All solvents and other reagents were words, this kind of polymer is called dual-ion conducting system. In this analytical reagent grade and used without further purification. case, the fast reaction of electrodeposition of Li+ on Li metal will lead to the concentration polarization in the polymer, and Li dendrites will 2.2. Preparation of Li-Nafion membrane, PEO-LiClO4 membrane and Li- be generated unavoidably [18,19]. Besides, solid electrolyte interface Nafion resin sol (SEI) would be generated because the solvent and lithium salts within the PEO polymer can also react with Li metal, which will reduce the The H-Nafion membrane was lithiated by dipping it in a 1 mol/L electrochemical performance of the battery due to the enlarged polar- LiOH aqueous solution and keeping at 80 °C for 12 h. After then, to ization and eventually cause the failure of the battery [6,15,20]. It is remove the LiOH solution, the lithiated membrane was treated at 80 °C also reported that the PEO complexing alkali metal salts can dissolve for 12 h in distilled water. The lithiated Nafion membrane is referred as the lithium polysulfides at high temperature, which will result in Li-Nafion membrane whose chemical structure is illustrated in Fig. S1b. “shuttle effect” [21]. Therefore, it is necessary to design single-ion The Li-Nafion membrane was vacuum dried 12 h at 80 °C and then was conducting polymer electrolytes to eliminate Li dendrites and reduce impregnated into PC for 1 h. The membrane surface was wiped by the dissolution of lithium polysulfides [19]. Besides, it is reported that nonwovens to remove the free PC on it before use. The thickness of Li- the Li-S cells fabricated with single-ion conducting polymer electrolyte Nafion membrane swollen with PC (noted as PC-Li-Nafion membrane) take a different reaction mechanism of sulfur reduction without gen- is about 30 μm. The PC-Li-Nafion membrane was served as separator erating lithium polysulfides [22,23]. and electrolyte in the Li-S cells. + The commercial Nafion membrane in H form (H-Nafion) (Dupont The PEO-LiClO4 membranes were prepared by mixing PEO and Co.), a kind of perfluorinated sulfonate ionomer, is widely applied in vacuum-dried anhydrous LiClO4 in acetonitrile under stirring. The proton exchange membrane fuel cells due to its excellent thermal and molar ratio of ethylene oxide segments to Li+ is controlled to be 20. chemical stability. After ion exchange of H+ with Li+, the lithiated After stirring for 24 h at room temperature, the mixed solution was cast Nafion (Li-Nafion) membrane swollen with propylene carbonate pos- onto a Teflon plate. After acetonitrile was evaporated at room tem- − − sesses the ionic conductivity of 8.18 × 10 5 Scm 1 at room perature, the membranes were finally dried under vacuum at 40 °C for

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