Binders

CHAPTER SIX

Binders The binder is an essential component because it plays an important role in maintaining the structural integrity of the electrodes, and thus the capacity and the ability to operate cyclic lithium-sulfur batteries. A good binder should create a good and stable network between sulfur and conductive materials, facilitating electronic and ionic transport [1]. According to [2], binders should aid in electrode processing and drying on to aluminum current collectors, electrolyte wetting during cell assembly, ion transport, and mechanical integrity upon cycling to accommodate the volume changes associated with S8-Li2S interconversion. Several alternative electrode binders, such as polyethylene oxide (PEO), gelatin, polyvinyl pyrrolidone (PVP), Na-alginate, gum arabic (GA), etc., have been studied as alternatives to common polyvinylidene difluoride (PVDF) [3–8]. According to [2], also PVP blends with Nafion, PAMAM dendrimers, polycationic-cyclodextrins, poly(acrylic acid), poly-(ethylene oxide), and carboxymethyl-cellulose:styrenebutadiene-rubber (CMC: SBR), have been investigated as the binders aimed to improve cathode performance. Some binders mitigated the migration of soluble polysulfides from the cathode into the electrolyte, which otherwise would lead to stranded sulfur in the cell or instabilities in the lithium anode. None has been reported that directly participated in the redox chemistry of sulfur or otherwise served to enhance ion transport as needed for high-rate applications. The ideal binder is needed to create more robust conductive structures and improve the strength and structural stability of the entire sulfur electrode. According to [9], capacity fading on cycling of lithium-sulfur batteries results from at least four processes: increase of SEI thickness resistance, loss of cathode capacity (precipitation of sulfur species outside the cathode), agglomeration and thickening of sulfur species, and increase in cell impedance because of reduction of the electrolyte. An important issue that has not been properly addressed up to now is the influence of the type and content of the cathode binder on the cell parameters and on the electrochemical performance of lithium-sulfur batteries.

Next-generation Batteries with Sulfur Cathodes https://doi.org/10.1016/B978-0-12-816392-4.00006-2

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PEO shows good ionic conductivity resulting from its coordination with lithium ions [10]. Using PEO as a binder in the cathode, 80% of the theoretical capacity is provided for the first discharge, and the capacity of 100 mAh g 1 is maintained at a temperature of 80°C. The morphology and electrochemical properties of the cathode depend on the processing procedure and the type of binder: a mechanically mixed mixture of PEO, carbon, and sulfur forms a porous PEO film that has better cyclic performance than the milled ball mill mixture that forms a dense and thick PEO layer over the sulfur particles. In contrast, when poly(butadiene-co-styrene) (PBS) serves as a tackifier in a ball mill, the dry mixture has a good ability to work cyclically [11]. In general, PEO exhibits poor adhesive properties [12, 13] and low temperature ionic conductivity [14], and electrodes made from it show better performance at elevated temperatures. The use of polytetrafluoroethylene (PTFE)-carboxymethyl cellulose (CMC) composites as binders improves the capacity and use of sulfur in comparison with polyvinyl pyrrolidone (PVP) and PTFE-polyvinyl alcohol (PVA) composite used as binders [15], but no cyclic performance is reported. In another work, the capacity increases, as noted, along with the number of cycles up to the 10th cycle, using a PTFE-CMC composite as a binder [16]. According to [17], the discharging capacity and cyclic work efficiency have been improved using a combination of PVP and polyethylene imine (PEI) compared to a single component PVP binder. The chemical reaction between PVP and PEI increases the stability of the PVP-PEI composite in the electrolyte, giving good mechanical strength to the cathode. Gelatin is a recently developed binder for a sulfur electrode. The cathodes utilizing gelatin as a binder have a better initial capacity and cyclic capacity than those using PEO as a binder [1, 18–20]. Gelatin exhibits favorable properties of high stability in electrolytes, effective adhesion, and dispersion of positive electrode materials. In [7], a binder inspired by Acacia Senegal, GA, is presented, which is nontoxic and balanced for high-performance lithium-sulfur batteries. The GA structure is shown in Fig. 22. Fig. 23A shows the discharging/loading profiles of the link with the binder GA at C/5. It was evident that there are two typical plateaus during discharge, corresponding to the form at ion of soluble long-chain polysulfides and insoluble short-chain Li2S2/Li2S. As shown in Fig. 23B, S@GA could provide an initial capacity of 1386 mAh g 1, which was significantly higher than that of S@Gelatin and S@PVDF. A large 1090 mAh g 1 capacity could still be reached after 50 cycles for S@GA, indicating the excellent

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Fig. 22 A schematic image of the GA chemical structure with Acacia Senegal background [7].

Fig. 23 (A) Cell discharging/loading profiles with GA binder at C/5; (B) the performance of cyclic work of cells with different binders [7].

integrity of the cell electrode from the GA. On the other hand, cell capacities with gelatin and PVDF decreased to 452 and 312 mAh g 1, respectively. Although these alternative positive binders affect lithium-sulfur batteries, there are still many limitations to their practical applications due to the unclear way these binders interact with the sulfur cathode to increase performance. Therefore, more research needs to be done to understand this mechanism. Some other modern binders are characterized in the following paragraphs. In [9] the electrochemical behavior, during prolonged cycling, of Li2Sbased cathodes containing five different binders was analyzed. These binders were: poly(vinylidene fluoride) (PVDF-HFP), polyvinyl pyrrolidone (PVP), mix of PVP with polyethyleneimine (PEI), polyaniline (PANI), and lithium polyacrylate (LiPAA). Sulfur utilization in the cathode followed

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the order of LiPAA > PVP:PEI > PVP > PVDF-HFP > PANI. Depending on the type of binder, cells provided 500–1400 mAh g(S) 1, 94.6%– 98.0% faradaic efficiency, and enabled more than 500 reversible cycles. In [21] a poly(acrylamide-co-diallyl dimethyl ammonium chloride) (AMAC) was introduced as the binder of high sulfur loading cathodes. AMAC, a cationic polyelectrolyte binder, was substantially insoluble in organic solvent while being highly soluble in water. Such features enabled retaining the stable void structure of the sulfur cathode. However, the ionic conductivity of the AMAC and sulfur aqueous slurry caused severe pitting corrosion of the Al current collector due to the formation of galvanic cells between sulfur particles and Al metal in the process of aqueous slurry coating. A dual-binder approach allowed effective elimination of the Al corrosion while still retaining the advantage of the AMAC binder. In [22], a redox-active supramolecular polymer binder was elaborated. It utilized a perylenebisimide (PBI) scaffold featuring four carboxylic acid moieties that can be lithiated by treatment with LiOH. Upon lithiation, the binder was water soluble, which permits aqueous processing, thereby maximizing the environmental and cost benefits of Li-S cells. Sulfur slurries containing the water soluble, lithiated PBI polymer binder were casted onto an Al foil current collector. Once casted, the binders self-assembled through stacking interactions of the aromatic cores into nanowire architectures. The obtained nanowire web morphology provided stronger physical binding between active S particles and the current collector. This avoided electronic disconnection between the electrode components, resulting in longer Li-S cell lifetimes. The lithiated nanowire structured electrodes demonstrated a 58% increase in capacity retention after 250 cycles at 1.5 C compared to nonlithiated control samples without modified nanowire architecture. In [2], -stacked perylenebisimide (PBI) molecules were utilized as highly networked, redox-active supramolecular polymer binders in sulfur cathodes for lightweight and energy-dense Li-S batteries. The in-operando reduction and lithiation of these PBI binders sustainably reduced Li-S cell impedance relative to nonredox active conventional polymer binders. That lower impedance enabled high-rate cycling in Li-S cells with excellent durability. Paper [23] presented a multifunctional polar binder (AHP) by polymerization of hexamethylene diisocyanate (HDI) with ethylenediamine (EDA) bearing many amino groups, which were used in electrode preparation with commercial sulfur powder cathodes. The abundant amide groups of the binder endowed the cathode with multidimensional chemical bonding

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interaction with sulfur species within the cathode to inhibit the shuttling effect of polysulfides, while the suitable ductility to buffer volume change. Utilizing such features, composite C/S cathodes based on the binder displayed excellent capacity retention at 0.5 C, 1 C, 1.5 C, and 3 C over 200 cycles. In [24], the molecular-level underpinnings distinguishing an active polyelectrolyte binder designed for lithium-sulfur batteries from a passive alternative were revealed. The binder, a cationic polyelectrolyte, was shown to both facilitate lithium-ion transport through its reconfigurable network of mobile anions and restrict polysulfide diffusion from mesoporous carbon hosts by anion metathesis, which is selective for higher oligomers. Such attributes allowed cells operation for >100 cycles with excellent rate capability using cathodes with real sulfur loadings up to 8.1 mg cm 2. According to [25], a sulfonated polystyrene (SPS) prepared via homogeneous reaction was used as a functional binder for the sulfur cathode of lithium sulfur cells. The traditional binder polyvinylidene fluoride (PVDF) was used for comparison. For the sulfur cathode with PVDF as binder, the capacity retention after 100 cycles at 200 mA g 1 was 46.9% and severe voltage fading performance from the 10th to 100th cycle occurred. For the SPS binder, the capacity retention after 100 cycles was 74.4% and there was almost no change of the first plateau at around 2.3 V in the discharge curve from the 10th to 100th cycle, indicating electrochemical performance improvement of the lithium-sulfur cell.

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