Harnessing Heteropolar Lithium Polysulfides by Amphoteric Polymer Binder for Facile Manufacturing of Practical Li-S Batteries

Lithium-sulfur (Li-S) battery is a cost-effective and sustainable energy storage technology owing to its good specific capacity, high energy density and being environmentally friendly. Nevertheless, its commercialization for large-scale applications is still a big challenge. This can be mainly attributed to fast capacity fading, low utilization of sulfur, safety concerns and slow reaction kinetics. These inherent performance issues are primarily associated with the insulation and volume changes between S and Li₂S, instability of the lithium metal anode and the shuttle effect of dissolved lithium polysulfides (LiPSs) intermediaries. Additionally, achieving durable and efficient charge storage requires lean electrolyte and high sulfur loading, which remain difficult to achieve.

Lately, research has focused on designing advanced sulfur hosts, 3D current collectors or multifunctional interlayers to enhance battery performance under these complications. While these methods hold great application potential, their inherent disadvantages and ineffectiveness at high sulfur loading make them generally unsuitable. To this end, addressing these issues and limitations through simple, scalable and compatible production methods is key to achieving technological readiness of Li-S batteries.

Polymer binders are indispensable components in battery electrodes due to their critical role in keeping the electrode structural integrity during the charging/discharging cycles. Although polar binders containing functional groups exhibit improved battery performance than commonly used polyvinylidene fluoride binders, they fail to provide adequate binding to the polysulfide anions and the long-chain LiPSs tend to aggravate during long-term cycling. Regardless, there is little research attention to manipulating electronegative sulfur atoms to solve the LiPS regulation problems. In addition, functional polymer binders that could allow high Li-S battery performance under lean electrolyte and high sulfur loading is yet to be developed as the underlying mechanisms are poorly understood.

On this account, Wuhan University researchers: Dr. Shizhen Li, Mr. Wenshan Xiao, Professor Hainam Do, Dr. Hangqi Yang, Dr. Xiaoqi Xu and Professor Chuang Peng presented a new rational design of an amphoteric polymer binder with both positively and negatively charged moieties for facile, scalable and robust fabrication of practical Li-S batteries. The physical properties and rich intermolecular interactions of the binder and their abilities to address the inherent limitations in Li-S batteries were evaluated and discussed. The work is currently published in the journal, Small.

The research team reported that the amphoteric polymer binders played a critical role in creating a uniquely charged environment to facilitate the coregulation of lithium cations and LiPSs as well as intermolecular interaction between the electronegative sulfur atoms and electropositive lithium atoms in LiPSs. The facile blending method endowed the mixed polymer with desired physical properties and processability, resulting in significant improvement in polysulfide adsorption, lithium-ion transportation, catalysis, anode stability and cathode robustness, even under rigorous conditions.

At relatively lower sulfur loading and copious electrolyte, the cell exhibited a lower capacity-fading rate of about 0.056% cycle-1 upon 700 cycles. At low electrolyte content to sulfur lading ratio of 6µL mg^‑1 and sulfur loading of 6.8 mg cm^‑2, the cell continued to yield stable areal capacities of 4.2 – 4.8 mAh cm^-2 in 50 cycles without noticeable decay at 0.02 C. The minimized LiPS shuttling was beneficial in improving the stability of the battery, while the catalytic effect improved sulfur utilization. Furthermore, an improvement in rate performance and subsequent reduction in the voltage hysteresis was observed.

In summary, Wuhan University researchers successfully designed a new multifunctional cationic polyacrylamide and hyperbranched polyester mixed polymer as a cathode binder for Li-S batteries. By addressing the multiple issues in Li-S batteries, the binders enhanced the overall performance of the batteries even at lean electrolytes and higher sulfur loading. The commercial practicability and feasibility of this work was further demonstrated by its zero added weight, ease of manufacturing and scalability and low production cost. In a statement to Advances in Engineering, Professor Chuang Peng explained their new findings would contribute to improving the technology and manufacturing readiness of Li-S batteries for commercial applications.