Hybrid Electrolyte and Bi-Functional Cathode Binder for Lithium-Sulfur Batteries
Authors: Bhattacharya Priyanka , Kumar Jitendra , Nandasiri Manjula I , Henderson Wesley A , Xiao Jie , Subramanyam Guru 
Published: February 2020

Lithium-sulfur (Li-S) batteries are promising energy-storage systems, offering up to five-fold increase in energy density as compared with present state-of-the-art Li-ion batteries thus potentially serving as a means to meet the growing demand for safe, environmentally friendly, high-energy density applications. For commercial batteries, cathodes with a high sulfur (active material) loading (> 2 mg/cm2) will be essential. Most reports on engineered cathode materials for Li-S batteries, however, are based upon low sulfur loadings (typically ~1 mg/cm2), which are impractical and often give misleading results about performance characteristics. It is unknown how such cathodes behave under high sulfur loading conditions. The binder is perhaps the most critical material for achieving a high sulfur loading. We have recently used polyamidoamine (PAMAM) dendrimers with differing surface chemistries as functional binders in Li-S cells with SuperP/S as the cathode material. Without any special efforts to engineer the cathode, we were able to obtain very favorable cycling stability and electrolyte wetting. This was attributed to the high density of the surface functional groups on the dendrimers which results in a larger number of possible binder-S/carbon interactions, and high curvature of the binder and high porosity which allows the sulfur particles to be more extensively exposed to the electrolyte (thus enhancing wetting). In addition, the binding between the large number of oxygen atoms on the dendrimers and the polysulfides, along with the greater number of interior pores, aids in preventing the dissolution of the polysulfides in the organic electrolyte. Dendritic macromolecules are also much more polar than linear conventional polymer-based binder chains and have a uniform distribution of surface groups. This ensures better interfacial interactions between the functional groups and carbon/S particles, as well as better adhesion to the substrate.
Alongwith the cathode, the electrolyte is another critical component of Li-S batteries as it regulates the polysulfide shuttle phenomena thus determining the rate capability, cycling stabilities and energy densities of the batteries. The safety of the battery is also governed to a large extent by the electrolyte.
Here, we will discuss the fundamental characteristics of dendrimers as aqueous binders for sulfur cathodes and compare their performance with other aqueous, commonly used linear polymeric binders such as styrene butadiene rubber (SBR) and sodium carboxyl methyl cellulose (CMC). Specifically, generation 4 PAMAM dendrimers with hydroxyl, 4-carbomethoxypyrrolidone, and sodium carboxylate surface functional groups served as promising, electrochemically stable binders for high sulfur loadings (~3-4 mg/cm2) with high initial cathode capacities (> 1000 mAh/g). In comparison to the use of a CMC-SBR binder, the electrodes with the dendrimer-based binders showed greatly improved performance. The CMC-SBR binder-based electrodes failed at high C-rates (0.2C) after 40 cycles, whereas the electrodes with the dendrimer-based binders had a capacity retention of >85% for more than 100 cycles. A detailed physicochemical characterization of the electrodes will be presented to substantiate the superior dendrimer-carbon/S interactions. We will also discuss our recently developed lithium aluminum germanium phosphate (LAGP) based solid-polymer-liquid hybrid electrolyte-separator system in conjunction with the above promising cathode materials for Li-S batteries. The hybrid separator showed (i) higher liquid electrolyte uptake, (ii) higher room T ionic conductivity, (iii) lower interfacial resistance with lithium, and (iv) lower cell voltage polarization during lithium cycling at high current density of 1.3 mA/cm2 at room temperature. The enhanced performance is attributed to higher liquid uptake, LAGP-assisted faster ion conduction and dendrite prevention. It is anticipated that by using novel dendrimer chemistries for electrode assembly, and hybrid electrolyte systems, this research will lead to the development and commercialization of high energy density, safe and long-cycle life Li-S batteries.

Figure 1