Lithium-sulfur (Li-S) batteries are regarded as promising candidates to replace lithium-ion batteries (LIBs) for electric vehicles and portable electronic devices due to the high theoretical energy density, low cost, and environmental benignity. However, the practical applications of Li-S batteries are still hindered by several issues including the sharp capacity decay caused by the lithium polysulfides (LiPs) shuttle effect, sluggish electrochemical redox kinetics, and detrimental Li dendrite growth. In this thesis, the primary objective is to enable practical high energy and long-cycle life Li-S battery by nanostructured electrode engineering and novel electrolyte design to address the above-mentioned issues.

The thesis begins with sulfur cathode engineering to relieve the polysulfide...[ Read more ]

Lithium-sulfur (Li-S) batteries are regarded as promising candidates to replace lithium-ion batteries (LIBs) for electric vehicles and portable electronic devices due to the high theoretical energy density, low cost, and environmental benignity. However, the practical applications of Li-S batteries are still hindered by several issues including the sharp capacity decay caused by the lithium polysulfides (LiPs) shuttle effect, sluggish electrochemical redox kinetics, and detrimental Li dendrite growth. In this thesis, the primary objective is to enable practical high energy and long-cycle life Li-S battery by nanostructured electrode engineering and novel electrolyte design to address the above-mentioned issues.

The thesis begins with sulfur cathode engineering to relieve the polysulfide shuttle effect and enhance sluggish electrochemical redox kinetics. A polar yolk-shell Co-Fe mixed metal sulfide (FeCoS₂) sulfur host with good electric conductivity is developed to improve the LiPs affinity of the host material. In addition, an ordered macroporous host material is developed to enhance ion transportation under high sulfur content and facilitate the growth of Li₂S-electrolyte-carbon triple-phase boundary to further boost the sulfur redox kinetics. Meanwhile, double-end binding site composed of polar material and single-atom catalyst is introduced into the ordered macroporous host to relieve the shuttle effect and catalyze the sulfur redox to improve the cycling and rate performance. An Ah-level pouch cell with the engineered cathode delivers high specific energy (>300 Wh kg^-1) with a high capacity retention rate of 74% for 80 cycles.

To modify the electrode electrochemical behavior during cycling, thus simultaneously enabling a robust solid electrolyte interphase (SEI) formation and relieving LiPs shuttling, this thesis then focuses on electrolyte design for Li-S batteries. A highly fluorinated ether-based electrolyte is proposed to enable the formation of robust LiF-rich SEI on the surface of Li metal anode to modify the Li striping/plating processes and avoid the formation of dendritic Li during cycling. Meanwhile, the low solubility of polysulfide in the fluorinated ether can effectively prevent the flooding of polysulfide, which can relieve the capacity decay caused by polysulfide shuttling. By applying this electrolyte, a Li-S cell with high S loading (4.5 mg cm^-2) and low electrolyte/sulfur ratio (10 μL mg^-1) can deliver a high areal capacity (> 3 mAh cm^-2) and high coulombic efficiency (>99%) for 100 cycles.

To control the negative-to-positive capacity ratio (N/P ratio) and improve the anode cycling stability of the battery, nanostructured Si-based materials are proposed as lithium storage anodes. In this thesis, a carbon encapsulated Si nanoparticle is developed to improve the electric conductivity, control the SEI growth, and enhance the structural stability of Si-based anode. Moreover, an integrated microsize Si anode is developed to improve the tap density of Si-based anode. The as-prepared Si anode delivers a high specific capacity (> 1500 mAh g^-1) and a high capacity retention rate (88.43%) for 100 cycles. Meanwhile, under high Si loading (2.5 mg cm^-2), the nanostructured Si anode can deliver a high areal capacity (> 3.0 mAh cm^-2) with a high capacity retention rate of 79.5% for 100 cycles.

Keywords: lithium-sulfur batteries; pouch cells; solid electrolyte interphase; lithium dendrite; fluorinated electrolyte.