A multi-functional sulfurized polyacrylonitrile interlayer for lithium sulfur batteries

A multi-functional sulfurized polyacrylonitrile interlayer for lithium sulfur batteries

Journal Pre-proofs A multi-functional sulfurized polyacrylonitrile interlayer for lithium sulfur batteries Fei Wang, Xuan He PII: DOI: Reference: S01...

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Journal Pre-proofs A multi-functional sulfurized polyacrylonitrile interlayer for lithium sulfur batteries Fei Wang, Xuan He PII: DOI: Reference:

S0167-577X(19)31228-5 https://doi.org/10.1016/j.matlet.2019.126604 MLBLUE 126604

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Materials Letters

Received Date: Revised Date: Accepted Date:

10 June 2019 26 August 2019 30 August 2019

Please cite this article as: F. Wang, X. He, A multi-functional sulfurized polyacrylonitrile interlayer for lithium sulfur batteries, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.126604

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© 2019 Published by Elsevier B.V.

A multi-functional sulfurized polyacrylonitrile interlayer for lithium sulfur batteries

Fei Wanga, Xuan Hea*


Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China


Functional interlayers applied in lithium sulfur batteries have been widely investigated due to its high reutilization of the diffused polysulfides and good cycle stability. However, the functional layer on the modified separator will indirectly decrease the sulfur content, which is harmful to the energy density of the whole cell. Herein, we report sulfurized polyacrylonitrile (SPAN) modified functional separator which can deliver quite a lot reversible capacity to overcome the shortage of traditional interlayer. Meanwhile, S and N dual dopants can offer abundant adsorptive sites and have strong chemisorption to polysulfides. Thus, with this modified separator, a facile sulfur cathode can deliver a high initial discharge capacity of 1338 mAh/g and merely 0.178% capacity fade per cycle over 200 cycles at 1C. The results indicate that SPAN is a promising material for functional interlayer between cathode and separator. Keyword:energy storage and conversion; functional; nanocomposite

1. Introduction

Lithium sulfur batteries are very promising candidate for next generation energy storage devices due to its potential low-cost and high capacity. However, the large-scale application of this ideal material is hindered by the poor conductivity of sulfur and diffusion/migration of intermediate 1

polysulfides [1]. Thus extensive efforts have been made to solve the problems above, including confining the sulfur within the various structure carbon host to improve the conductivity of sulfur cathode [2], or doping different atoms such as N, S, O and B into the carbonaceous host material to suppress the migration of polysulfides [3]. Many strategies required a sophisticated synthesis procedure in order to achieve a good electrochemical performance[4]. Under this circumstance, a functional interlayer between cathode and separator, as a novel strategy, was raised which could deliver an excellent electrochemical performance even with a simple carbon/sulfur cathode. This is because the interlayer acts as an accessorial current collector to reutilize the polysulfides and a physical shield to block the migration of polysulfides [5]. Whereas, most interlayers that have been reported[5-7] had few reversible capacities during the charge and discharge process, which would indirectly decrease sulfur content and be harmful to the battery energy density. Herein, we firstly report sulfurized polyacrylonitrile (SPAN) modified separator which can not only suppress the shuttle effect through strong chemisorption of N, S elements and improve the utilization of active material, but also deliver quite a little capacity. With this SPAN modified separator, a simply sulfur/acetylene black composite with 3.2mg/cm2 sulfur loading exhibited an initial discharge capacity of 1338mAh/g and 65% capacity retention at 1C. Moreover, the SPAN interlayer could deliver a very steady discharge capacity at the cut-off voltage range from 1.8-2.6V.

2. Experimental section

2.1 Synthesis of SPAN interlayer and cathode

All used chemicals were of analytical grade and without any further treatment. Sulfur and polyacrylonitrile (PAN) were mixed through ball milling at 500 rpm for 10 h with the weight ratio 2

of 4:1; then the mixture was heated at 155°C for 2h and 300°C for 6h under the protection atmosphere of N2. Extra sulfur would be evaporated in 300°C and only SPAN was left in the reactant. The SPAN interlayer, coated on the single side of the separator, was consisted of prepared SPAN, acetylene black and Carboxymethyl Cellulose (CMC) binder in the weight ratio of 8:1:1 through doctor blade method. After sufficient dry in the vacuum oven at 60 ℃, the modified separator and was cut into wafers with a diameter of 12 mm for cells assemble. The cathode active material was synthesized via mixing sulfur and low-cost acetylene black in the weight ratio of 4:1 and then heating at 155°C for 12 h under the Ar atmosphere. The typical SPAN and sulfur loading are about 0.88 mg/cm2 and 3.2 mg/cm2. Meanwhile, the size of cathode and anode was a disk with a diameter of 12 mm and 15mm respectively. 2.2 Electrochemistry measurement The CR 2032-type coin cells were assembled in an Ar-filled glove box and tested via LANDCT2001A testers and CHI660E electrochemical workstation at room temperature. The lithium metal was selected as the anode. Moreover, the employed routine separator was Celgard 2400. The employed electrolyte in each tested cell was 30 μm typical ethers electrolyte consisted of 1 M LiTFSI in DME and DOL (volume ratio 1:1) with 0.1 M LiNO3. 3. Result and discussion As showed in Fig. 1a, the as-obtained SPAN particle has a uniform size distribution with a diameter of 200 nm. Element mapping (Fig. 1b-c) demonstrate the homogeneous distribution of N and S in SPAN molecule. According to the XRD patterns of SPAN in Fig. 1d, no signature of sulfur was observed, indicating complete reaction between S and PAN.

It can be seen from Fig. 1e that

a dense coating layer, containing nanosized SPAN particles, attaches to one side of the Celgard 2400 3

separator. The thickness of SPAN interlayer is ~ 23.8 μm, which have a similar thickness with other functional interlayers [6,7]. However, unlike these functional interlayers, 51.2% sulfur content SPAN interlayer is able to deliver the capacity that could not be ignored. To evaluate the electrochemical performance of SPAN modified electrode, the coin cells with and without the modified separator were tested. Fig. 2a displays the CV curves for the cell with SPAN modified electrode. The cathodic peak at 2.31 V and 2.06 V corresponding to the reduction of sulfur into long-chain soluble polysulfides, and then to insoluble short-chain polysulfides. Meanwhile, in the anodic process, there are two oxidation peaks at 2.28V and 2.35 V, which should be the reverse reaction of polysulfides to sulfur [6]. Obviously, there are some extra capacities in the oxidation and reduction process which should be the contribution of SPAN. Fig. 2b displays the voltage-specific capacity curves for the electrode with a coated separator at different current density. With the increment of current density, the discharge platforms drop. But the two discharges platform could still be observed, which indicating a good electronic conductivity of SPAN interlayer. According to Fig.2c, with the modified separator, the cell is able to deliver a discharge capacity of 1692, 1474 1322, 1021 and 657 mAh/g at 0.1, 0.2, 0.5, 1 and 2C respectively, which is much higher than the cell with a routine separator. Long cycle performance of SPAN modified separator electrode has been studied in Fig. 2d at the current density of 1C. It achieves an initial discharge capacity of 1338 mAh/g ant retains 871 mAh/g after 200 cycles, displaying good capacity retention and cycle stability. Generally, 23.8 μm SPAN interlayer would increase the distance of electron transfer and rise the impedance of the cell[8]. However, in Fig.2e, according to the semicircle in the high to medium frequency range, the SPAN modified separator exhibited a lower charge transfer resistance than the routine separator, which could ascribe to the redistribution of sulfur and good 4

conductivity of SPAN itself[9]. With the purpose to directly perceive the chemical interaction between SPAN molecule and polysulfides, 16 mg Li2S4(Aladdin) was mixed with 4 ml typical electrolyte. And then 0.05g acetylene black and SPAN powder was added into the solution. According to Fig.2f, the solution with SPAN was much clear than other groups, which proved a good chemical adsorptive behavior of SPAN. In order to further investigate the specific capacity contribution of SPAN modified electrode, cells were assembled without sulfur/carbon cathode but only SPAN modified separator. Moreover the specific discharge capacity was calculated based on the weight of sulfur cathode. According to Fig. 3a-c, the cells with the cut-off voltage range from 1V to 3V, which is the most typical cut-off voltage for SPAN electrode [10], have a higher discharge capacity than those cycle from 1.8V to 2.6V at the first 30 cycles, as the SPAN fully react with the Li anode [11]. However, in ether electrolyte, fully reaction between Li+ and sulfur in SPAN molecule chain would broke the sulfurcarbon bond and caused fast capacity decay[12]. Thus, shorter cut-off voltage exhibits much better capacity retention and cycle stability than the longer one. The SPAN modified electrode can achieve a highly reversible capacity of 112 mAh/g with only 0.12% capacity fading per cycle under the current density of 1C. 4. Conclusion In summary, we have demonstrated that SPAN interlayer can effectively improve the utilization of sulfur, suppress the shuttle effect of intermediate polysulfides and offer quite a lot capacity. A facile sulfur/acetylene black cathode (3.2 mg/cm2 sulfur loading) can deliver a high initial capacity of 1338 mAh/g and 65 % capacity retention after 200 cycles at 1C. Meanwhile, SPAN interlayer displays much better reversibility at the cut-off voltage range from 1.8V to 2.6V (0.12% capacity 5

fading per cycle) than 1V to 3V (0.56% capacity fading per cycle). The present strategy provides a new avenue for the development of high-performance lithium sulfur batteries.


This work was supported by the National Key Research and Development Program of China (No.2016YFB0300801), Major Research Equipment Development Projects of the National Natural Science Foundation of China (No. 51327902).


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Fig. 1. The physical characteristics of synthesized SPAN particle: (a) SEM images of SPAN particle, EDX elemental mappings of SPAN (b) Sulfur and (c) Nitrogen;(d) XRD pattern of SPAN and Sulfur; (e) the cross-section SEM images of SPAN modified separator.

Fig. 2. (a) Initial four-cycle curves of CV at scan rate of 0.05 mV/s with SPAN modified separator. (b) Voltage vs. specific capacity profiles at various rates with SPAN modified separator. (c) Discharge capacity at various rates and (d) long-term cycle stability of SPAN modified separator at 1C. (e) Electrochemical impedance spectra SPAN modified separator and routine electrode. (f) Static adsorption of Li2S4 by various host material.

Fig. 3. (a) the cycle stability of only SPAN modified separator at different cut-off voltage, (b) and (c) voltage vs. specific capacity profiles at the cut-off voltage of 1.8V to 2.6V and 1V to 3V.


A multi-functional sulfurized polyacrylonitrile interlayer for lithium sulfur batteries Fei Wanga, Xuan Hea* a State

Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Highlights  High sulfur content SPAN was synthesis via a facile method  A novel material (SPAN) was first applied into the functional interlayer  A SPAN modified electrode can deliver a discharge capacity of 1335mAh/g at 1C  Suitable cut-off voltage of SPAN in typical ethers electrolyte has been studied



Declaration of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled