Exploring advanced binder and electrolyte system for high-performance lithium-sulfur batteries

Abstract

Lithium-Sulfur (Li-S) batteries have received significant attention as promising alternative to lithium-ion batteries due to their high theoretical specific capacity (1675 mAh·g-1Sulfur) and energy density (2500 Wh·kg-1). However, there are fundamental challenges impeding their commercialization. Notorious among those challenges is the “polysulfide shuttle” consisting of the dissolution into the electrolyte and subsequent crossover to the negative electrode of long-chain, high-order sulfides (LiPSs). The mitigation of this polysulfide shuttle through electrode engineering and electrolyte design is key to realizing durable Li-S cells. In this thesis, successive studies are conducted with the goal of contributing novel insights and approaches for overcoming this fundamental barrier. ☐ The lesser-known role of the conventional binder (polyvinylidene fluoride, PVDF) in the instability and degradation of sulfur electrodes in Li-S batteries, as well as potential strategies for its mitigation were investigated. In addition, the influence of various electrolyte systems on the reaction pathways governing polysulfide dissolution and migration has been systematically investigated. Based on these findings, a biphase electrolyte system is proposed to eliminate LiPSs-shuttle and enhance the electrochemical stability of Li-S batteries. ☐ The first study investigates a critical failure pathway in PVDF-based sulfur electrodes that have not been widely recognized. It is revealed that 1,3-dioxolane, a common solvent in Li-S electrolytes, falls within the Hansen solubility sphere of PVDF, leading to the gradual dissolution of PVDF. Consequently, this dissolution compromises electrode structural integrity, increases electronic impedance, and causes rapid loss of active sulfur via the LiPS shuttle. To address this issue, a novel double crosslinked starch (DCS) binder was synthesized via two cross-linking reactions and contrasted against PVDF to mechanistically highlight the impact of such PVDF-dissolution. Compared to PVDF, DCS remains insoluble in the electrolyte, preserving the structural integrity of the sulfur electrode and exhibiting 2.5 times lower impedance after 200 cycles. Operando Raman spectroscopy, visual cell observations, and ex-situ UV-vis analysis confirmed superior polysulfide retention of the DCS-bound electrode. It achieved a discharge capacity of 522 mAh·gs-1 at 0.1 C rate after 200 cycles, representing a capacity fade of 0.14% per cycle beyond the initial 20 cycles. Under similar conditions, PVDF-bound sulfur electrode delivered a lower discharge capacity of 434 mAh· gs-1, and a higher decay rate of 0.24% per cycle after stabilization. This study highlights the limitations of PVDF binder in S-electrodes and investigates an alternative candidate (DCS) binder, which improves the structural integrity of S-electrodes and demonstrates superior electrochemical performance. ☐ The second study explores the influence of different electrolyte paradigms on the dissolution and diffusion of LiPSs in Li-S batteries. These electrolytes are categorized into fully solvating electrolytes (FSEs) and sparingly solvating electrolytes (SSEs). A comprehensive literature meta-analysis reveals that the most significant benefit of sparingly solvating electrolytes in Li-S batteries is the improvement in coulombic efficiency. Optical operando Li-S cell and ex situ UV-vis analysis were used to investigate polysulfide speciation in FSE and SSE. Experimental optical imaging enables real-time visualization of polysulfide dynamics, while UV-vis spectroscopy facilitates the identification of soluble polysulfide species within the electrolyte at the completion of cycling. Optical imaging and UV-vis characterization reveal that an increase in lithium salt concentration in the electrolyte, which renders it more sparingly solvating, induces a shift toward the formation of shorter-chain polysulfides. The transition to shorter-chain polysulfides indicates a reduction in the polysulfide species participating in LiPS-shuttle, thereby enhancing the coulombic efficiency. Under similar conditions, FSE follows the well-known reaction pathway, where sulfur initially converts to long-chain LiPS before transitioning to short-chain LiPS. This study demonstrates that SSEs minimize LiPS-shuttle, which is a major contributor to capacity fading and poor coulombic efficiency in FSEs. ☐ The final study proposes a novel biphase electrolyte based on the phase separation between solvents with different polarity. The biphase electrolyte decouples the cathode electrolyte (catholyte) from the anode electrolyte (anolyte), forming a selective interfacial membrane between them. This membrane blocks LiPS crossover from the catholyte to the anolyte, while allowing Li+ ion transport. Hansen Solubility Parameters of solvents are used to identify non-aqueous solvent pairs that exhibit and maintain phase separation during the cycling of Li-S batteries at room temperature. Candidate solvent pairs exhibited significant differences in their partial polarities. The high-polar solvent was used to prepare the catholyte due to its strong LiPS solvation power, while the low-polar solvent was used to prepare the anolyte. As a proof-of-concept, cycling an operando visual cell with biphase electrolyte confirms that the selective interfacial membrane effectively confines LiPSs to the catholyte. Electrochemical cycling further demonstrates that the biphase electrolyte maintains a more stable coulombic efficiency after the initial cycles compared to conventional electrolyte, indicating effective suppression of LiPS-shuttle. This study presents a new pathway for advancing the development of high-performance Li-S batteries. ☐ Together, these studies highlight the importance of developing alternative binders for S-electrodes, elucidate the reaction mechanisms governing polysulfide speciation in an electrolyte that reduces LiPS-shuttle, and propose a novel electrolyte that effectively suppresses LiPS-shuttle.