Advanced Strategies towards High Performance Lithium-Sulfur Batteries

As one of the most promising energy storage devices, the commercialization lithium-sulfur batteries (LSBs) are still facing obstacles due to the notorious shuttling of soluble polysulfide intermediates, accompanied by low S utilization, Li dendrite growth, and volume expansion during charge/discharge process. In this thesis, we explore various approaches to overcome the challenges and achieve high-performance LSBs. The modified separator can serve as a barrier to alleviate the polysulfide shuttle efficiently. Here, we introduce an electronic and ionic co-conductive separator coating composed of multi-walled carbon nanotubes/lithium lanthanum titanium oxide (MWCNTs/LLTO), aiming at improving the overall electrochemical performance of LSBs. The proposed MWCNTs/LLTO-modified separator improves the redox reaction kinetics from soluble higher-order lithium polysulfides (LiPSs) to the insoluble lower-order ones and ultimately to Li2S, thereby reducing the LiPSs dissolved in the electrolyte. It also serves as a physical barrier to adsorb LiPSs, efficiently preventing their diffusion from the cathode to the anode. LSBs adopting the MWCNTs/LLTO-modified separator exhibit higher ionic and electronic conductivity than un-modified counterparts, leading to an initial specific capacity and excellent rate capability performance. Solid-state electrolytes (SSEs) are attracting extensive interest for next-generation LSBs due to excellent thermodynamic/electrochemical stability. We use in-situ polymerized poly(1,3-dioxolane) (PDOL) to solve the poor interfacial contact between SSE and electrodes. SnF2, as a catalyst, assists LiPF6 in the rapid polymerization of DOL at room temperature and enhances interfacial stability by forming a hybrid LiF/Li-Sn alloy interphase layer. This interface layer can promote a uniform distribution of Li ions and effectively inhibit the formation of disordered lithium dendrites. Consequently, the PDOL initiated by SnF2/LiPF6 demonstrated enhanced ionic conductivity of 3.12 ×10−5 S cm−1, a high lithium transport number of 0.82, and more stable electrochemical stability compared with the PDOL initiated by LiPF6. The cells with SnF2/LiPF6-PDOL electrolyte achieved an improved rate and cycling performance compared to those with LiPF6-PDOL electrolyte. To further understand the effect of SEI formed by the reaction between SnF2 and Li in DOL, it is introduced a composite SEI incorporating inorganic LiF/Li-Sn alloy with polymerized PDOL on the lithium metal anode. This composite structure was proved to effectively prevent the side reactions between highly reactive lithium metal and the electrolyte, reducing electrolyte consumption and mitigating lithium metal corrosion in liquid electrolytes. Additionally, the formation of inorganic LiF and Li-Sn alloy enhances mechanical strength of SEI, inhibiting Li dendrite growth and promoting uniform Li ion deposition. The symmetric cells show highly reversible plating/stripping at a high current density of 5 mA cm-2 due to enhanced interfacial stability. When coupled with either LiFePO4 (LFP) or S cathode, the cells exhibit improved rate and cycling performance compared to bare Li anode and Li anode with only inorganic SEI. The Li||LiFePO4 cells with this composite SEI exhibit enhanced long-term cycling, with a capacity retention of 85 % after 2000 cycles at 2.0 C. These studies offer some facile approaches for inhibiting polysulfide shuttle and protecting lithium metal anode, which provide more possibility to achieve the high performance of LSBs and expand the application of LSBs in the future.