A Novel Polar Copolymer Design as a Multi-Functional Binder for Strong Affinity of Polysulfides in Lithium-Sulfur Batteries
- Yu Jiao†1, 2,
- Wei Chen†2,
- Tianyu Lei†2,
- Liping Dai2,
- Bo Chen3Email author,
- Chunyang Wu2Email author and
- Jie Xiong2Email author
© The Author(s). 2017
Received: 12 February 2017
Accepted: 22 February 2017
Published: 16 March 2017
High energy density, low cost and environmental friendliness are the advantages of lithium-sulfur (Li-S) battery which is regarded as a promising device for electrochemical energy storage systems. As one of the important ingredients in Li-S battery, the binder greatly affects the battery performance. However, the conventional binder has some drawbacks such as poor capability of absorbing hydrophilic lithium polysulfides, resulting in severe capacity decay. In this work, we reported a multi-functional polar binder (AHP) by polymerization of hexamethylene diisocyanate (HDI) with ethylenediamine (EDA) bearing a large amount of amino groups, which were successfully used in electrode preparation with commercial sulfur powder cathodes. The abundant amide groups of the binder endow the cathode with multidimensional chemical bonding interaction with sulfur species within the cathode to inhibit the shuttling effect of polysulfides, while the suitable ductility to buffer volume change. Utilizing these advantageous features, composite C/S cathodes based the binder displayed excellent capacity retention at 0.5 C, 1 C, 1.5 C, and 3 C over 200 cycles. Accompany with commercial binder, AHP may act as an alternative feedstock to open a promising approach for sulfur cathodes in rechargeable lithium battery to achieve commercial application.
The lithium-sulfur (Li-S) rechargeable battery cells offering a theoretical cathode specific capacity of 1675 mAh g−1, which is five times higher than those commercial lithium ion batteries (LiCoO2 and LiFePO4), have been applied in a variety of the most promising energy storage devices to address the increasing energy storage demands for various technological applications [1–3]. Unfortunately, despite its considerable advantages, its practical use has been frustrated by several problems [4–6]. (1) The sulfur is low electron conductivity (5 × 10−30 S cm−1 at 25 °C), which generally causes low utilization of active materials. (2) Large variation in volume occurs during charge-discharge cycling, corrode cathode where they are not recycled on charge. (3) “Shuttle effect”, another major problem caused by the high dissolution of the discharge/charge, intermediates in organic electrolytes. Polysulfides dissolve into the electrolyte and penetrate through the separator to the lithium metal anode and then they are reduced to solid precipitates (Li2S), leading to quick capacity decay with the loss of active materials and an additional problem of low Coulombic efficiency in a rechargeable Li-S battery [7–10]. Although various approaches have been employed to overcome this problem [11–14], such as N-doped materials [15–17], carbon-based materials , conductive polymers [19, 20], metal oxides [21–23], and transition metal disulfides . None have proven commercially viable due to its high cost and not suitable for large scale manufacturing.
The binder is an important ingredient in Li-S battery, it functions to bond and keep the active materials in the electrode, to ensure well electrical contact between the active materials and conductive carbon, as well as to link the active materials with the current collector [24–28]. In particular, the recent investigations on silicon anodes have revealed that ideal binder should not only able to adhesion strength and ductility with inexhaustible tolerance of large volume change and still physically and/or chemically trap capacity to attain high initial reversible capacity and excellent cycle ability. Polyvinylidene fluoride (PVDF) is widely used as a conventional binder for Li-S batteries . However, due to the linear-molecular structure, PVDF just play a role of physical adhesion, enabling the mechanical linkage of the active materials with additives, the function will fade with time when there is no bonding between those polymers and the carbon substrate, resulting in the vexed problem that polysulfide dissolved in the aprotic electrolyte. Therefore, new functional binder is the urgent needs to make up for the deficiencies of PVDF. Thus it can be seen crosslink structured binder for further improvement in the cycle life is constructed with the increased number of active sites between polysulfides and binder. Recent investigations shown that functional materials endued with amine groups have been viewed as an ideal hunter to absorb polar lithium sulfides and nonpolar carbon surface, which effectively prevents loss of active mass during cycles [29, 30].
Hence, in this paper, we introduced a multi-functional AHP binder with plenty amide groups as an efficient binder for Li-S batteries. Strong interactions between discharge products polysulfides are created throughout the cathode by the unique amide/amino crosslink structures of designed binder to buffer the shuttle effect of sulfur cathodes. Unlike conventional polymeric binders (PVDF), obvious superiority of our design is the binder with interconnected polar structure to form a stable electrode, and exhibit ductile architecture, resulting in a marked improvement of conventional C/S cathode in cycle life. It is noteworthy that the presented strategy is not engineered in any specialized manner, thus making the process commercially viable.
Synthesis of the AHP Binder
Ethylenediamine (EDA), hexamethylene diisocyanate (HDI), and N,N-dimethylformamide (DMF) were purchased from Aladdin and used as received. The novel AHP binder was prepared by a copolymerize process using EDA (10 mmol) and HDI (5 mmol) in DMF solvent with high-speed magnetic stirring for 4 h at 60 °C. Then the product was uniformly dispersed in DMF solution with a mass ratio of 1 mg per 10 uL solvent.
X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra Dld, Japan) was used for elemental analysis and chemical bonding information after synthesis of EDA and HDI. Scanning electron microscope (SEM) was used to observe the surface topography of S cathodes with different binder before and after cycle.
Preparation of S@AHP Cathodes and Electrochemical Measurements
Bulk sulfur (Alfa Aesar, 043766) and acetylene black (Hefei Kejing Materials Technology Co., Ltd) with a mass ratio of 6:4 was ball milled for 60 min at 300 rpm. The obtained mixture was then heated at 210 °C for 12 h to encapsulate sulfur in the acetylene black. After cooling to room temperature, the C/S composite was obtained. Then the thermogravimetric analysis (TGA, SDT 2960, USA) was performed on an SDT 2960, TA Instruments to confirm the mass of sulfur. Typically, the preparation of electrodes and battery assembly were that the electrodes from the C/S composite were prepared by making slurry of C/S and AHP binder in a mass ratio of 8.5:1.5 in DMF solvent, respectively. The slurry was then casted on the surface of Al foil and dried under vacuum at 60 °C overnight. Electrodes contained approximately 0.5 mg of sulfur per square centimeter, and 30 uL electrolyte was used in a coin cell. For comparison, C/S cathodes with various binders were prepared using PVDF and PTFE (both from Hefei Ke Jing Materials Technology Co., Ltd) instead of AHP by similar route.
The electrolyte was 1 M LiTFSI dissolved in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 v/v) with 1wt% lithium nitrate (LiNO3) as additive. Cells were assembled in an argon-filled glove box, and the electrochemical test of discharge-charge properties and cyclic voltammetric were tested on the CT2001A cell test instrument (Wuhan LAND Electronic Co., Ltd) and CHI660E (Shanghai Chenhua instrument Co., Ltd) electrochemical workstation, respectively.
Results and Discussion
To better understand the effect of the mechanical and chemical characteristics among electrode components, the binding structural viewpoint of S with AHP and commercial linear binders (such as PVDF, PTFE) are expected to be different (Fig. 1c). During the discharge process (Li insertion), owing to the formation of insulating Li2S on the carbon matrix, linear PVDF or PTFE are forced to mechanical stretch or moved by the expanded polysulfides. The volume density of Li2S (1.67 g/cm3) is much lower than that of S (2.03 g/cm3), which would cause a volume expansion of the S cathode of ∼22% compared to their initial state during the whole discharge process. Upon charging (Li desertion) in the same cycle, the polysulfides shrink back to their original state; the linear binders, however, cannot fully follow the shrinkage of polysulfides, thus leading to an inevitably contact loss of electroactive materials from the carbon matrix, coupled with polysulfides dissolution, result in inferior performance of most sulfur-carbon composites. This issue becomes more prominent over extended cycling. On the contrary, AHP have plentiful amide side groups physically/chemically entangled to grasp polysulfides, leading to reinforced binding ability with polysulfides via hydrogen bonding [29, 30]. Thus, in this polymeric AHP binder provides multidimensional noncovalent interactions with the polysulfides surfaces through the amide groups. These interactions not only allow the AHP binder to accommodate the massive volume expansion of S cathodes during discharge process but also keep the polysulfides-binder interactions even during charge process.
Long-term cycling and well stability is the first goal of a commercial battery, the electrochemical stability of the C/S/AHP composite cathode was investigated by testing under 1 C for 100 cycles, compared with similar electrodes using PTFE and PVDF as binders. The cycle life, discharge capacity, and Coulombic efficiency of electrode with AHP as binder were significantly better than those with PTFE and PVDF. As shown in Fig. 3c, after 100 cycles, the capacity of AHP binder stabilizes at 628 mAh g−1 with 81.2% retention (Fig. 3d) at 1 C. It is shown, in our work, the capacities of the conventional PTFE and PVDF binders drop very fast. The discharge capacity of S@PTFE started at capacity of 728 mAh g−1 but degraded severely to 395 mAh g−1 after the same number of cycles, which corresponds to 54.3% capacity retention, and the PVDF binder dropped more severely with 47.3% capacity retention. The S@AHP electrode exhibits the best cycling performance compared with common binders in Li-S batteries when the commercial sulfur powder is taken as active material. The stable cycling performance and high Coulombic efficiency (99%) imply that the AHP binder is benefit to confine polysulfides in the eletrode. Over 200 cycles, the reversible capacities of the C/S@AHP electrode at different rates of 0.5 C, 1 C, 1.5 C and 3 C also show excellent stability (Fig. 3e). Apparently, the enhanced reversibilityof AHP binders contribute a lot to cyclic performance of S electrode a possible mechanism is that the plentiful amide groups could efficiently inhibit the leakage of polysulfide during cycling. The much improved performance of the C/S@AHP electrode was due to the fact that the polar amino group of the binder provides the strong affinity to absorb lithium polysulfide intermediates, resulting in enhanced cycling performance [29, 30].
In summary, we have successfully developed a polar binder with plenty amide groups as multidimensional bonding site for high-performance Li-S cells, making substantial progress in improving electrochemical properties and therefore resolving the chronic insufficient cycle lives of S cathode. We demonstrate the lots of amide functional groups of the AHP binder with high binding strength construct effectual trap the sulfur species and subsequently confine them within the cathode and inhibit the shuttling effect, while the excellent mechanical properties of the S@AHP cathode with suitable flexible to buffer the volume change of sulfur. When AHP was applied to assemble cells with commercial sulfur and acetlene black have been cycled, they could show the stable capacity retention at different rates of cycles. As a result, we believe the synthesis of this polymeric polymer will arouse the battery community’s interest in fabricating long life Li-S cells and provide a novel method for synthesis new materials for Li ion batteries.
This work was financially supported by the National Key Basic Research Program of China (2014CB931702), the National Natural Science Foundation of China (91421110), and Sichuan Provincial Fund for Distinguished Young Academic and Technology Leaders (2014JQ0011). Y. J., W. C., and T. L. contributed equally to this work.
JY and CW draft the manuscript. CW and LTY make the contribution on directing the experiments and data analysis. DLP, CB, and WCY have taken part in the acquisition and interpretation of the data. XJ formulates the idea of investigation and is the corresponding author of the work. All authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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