A Design of Lean-Electrolyte Lithium-Sulfur Cells

Abstract

Introduction As the demand for energy is harshly increasing nowadays, great effort has been devoted to developing various advanced energy-storage technologies with high energy density, long-term cyclability, and inexpensive costs. The lithium-ion battery technology has been the mainstream option for the portable energy supply for the past decades. However, the commercial lithium-ion battery has limited cathode capacity and energy density of about 170 mAh g-1 and 600 Wh kg-1, as well as the high costs of $10,000 per ton. To further increase the cathode capacity and lower the fabrication costs, sulfur, with a high theoretical capacity of 1,675 mAh g-1, a high theoretical energy density of 2,600 Wh kg-1 in lithium-sulfur batteries, and a cheap price of $150 per ton, has been chosen to become new cathode materials. Thus, lithium–sulfur battery, as an alternative branch of lithium-ion batteries, is a potential technology to solve the problems in portable energy supply.In lithium–sulfur system, multiple redox reactions occur during different steps of charge and discharge reactions. In the discharge process, the solid-state sulfur (S8) will be reduced to form liquid-state lithium polysulfides (Li2Sx, 4 ≤ x ≤ 8) and solid-state lithium sulfides (Li2S2/Li2S). The lithium sulfides will be charged back to solid-state sulfur. These reactions in charge and discharge processes repeat during cycling. Three key factors, the sulfur loading, the sulfur content, and the electrolyte-to-sulfur ratio, have been thoroughly researched for developing high-performance lithium–sulfur batteries in the past two decades. To commercialize the lithium–sulfur battery technology, limits on these three factors have been set, which are a sulfur loading of 5 mg cm-2 and above, a sulfur content of 65 wt% and above, and an electrolyte-to-sulfur ratio of 10 μL mg-1 and below. However, due to the complex reaction steps in lithium–sulfur system, these three factors are conflicted or cannot be achieved simultaneously in most of the research cases. Results and Discussion Here, in our presentation, we report a high-loading polysulfide cathode, achieving both high sulfur content and low electrolyte-to-sulfur ratio at the same time. A non-nanoporous carbon nanotube/nanofiber (CNT/CNF) is adopted as the electrode substrate to accommodate the active material, liquid polysulfide catholyte, forming a polysulfide cathode with a high sulfur loading of 8.64 mg cm-2, a high sulfur content of 68 wt%, and low electrolyte-to-sulfur ratios of 4–7 μL mg-1. With such a cathode design, the lithium-sulfur cell attain high discharge capacities of 630–870 mAh g-1 and 580–631 mAh g-1 at C/10 and C/5 cycling rates, which corresponds to the high energy densities of 10.5–11.5 and 11.4–15.7 mWh cm-2. In addition to the high electrochemical utilization, our lean-electrolyte lithium-sulfur cells demonstrate a long-term cyclability of 200 cycles.As a result, we further investigate the possible failure mechanism for a high-loading polysulfide cathode operating in a lean-electrolyte cell under a semi-dry electrolyte condition. The extended cycling test is performed to explore the discharge and charge efficiency as well as the lithiation and delithiation mechanisms. Two cycling rates, C/10 and C/5, are applied for comparison. It reveals that due to the semi-dry electrolyte condition, the decomposition of the polysulfide catholyte occurs during extended cycles. The overcharge issue is thus occurred as a result of the rapid consumption of electrolyte, and leads to slow ion diffusion, decreased Coulombic efficiency, and irreversible capacity loss. Additionally, the decayed cycling performances are more easily observed at the slow C/10 cycling rate than at the fast C/5 cycling rate. Conclusion In summary, according to the excellent cycling performances of our cell design with a high-loading cathode, high sulfur content, and lean electrolyte addition, we might be very close to overcome the three limiting factors for commercialization of lithium–sulfur battery technology. Moreover, the failure analysis of our lean-electrolyte lithium-sulfur cell demonstrates the main challenges in developing a lean-electrolyte lithium–sulfur cell. The attainment of a high and stable Coulombic efficiency as well as the fast electrolyte consumption during regular cycling rate would challenge the development of lean-electrolyte lithium-sulfur cell with more practical cell design parameters.