Sulfur-based cathodes operated at strict cycling conditions for Li-based batteries

Abstract

With widespread investments from the transportation sectors, the electrification of a significant portion of the transportation market appears to be on the horizon. However, the range and price of these vehicles remains a challenge and hinders the full market penetration of electric vehicles into the world markets. Although economies of scale can decrease the price of electric vehicles, at the core of the problem is the rising price of cobalt and the limited energy density of lithium ion batteries. Next-generation battery technologies that rely on sulfur-based cathode have great potential in terms of cheap raw materials and higher energy density. Such a technology is perfect for increasing the range and decreasing the cost of electric vehicles. Lithium-sulfur battery (the most common configuration of a sulfur-based cathode) has many technical challenges that hinder it practical application. The combination of the notorious polysulfide shuttle effect, electron insulating nature of reactants/products and volume expansion of active material, has rendered the cyclability of lithium sulfur batteries very poor. Indeed, much progress have been achieved in the recent years, but the low areal loadings and high electrolyte contents (relaxed testing conditions) used for these cells are too low to be of any practical significance. Lithium-sulfur batteries operated at strict conditions have challenges that are amplified when compared to their relaxed testing conditions (low loading/high electrolyte content) counterparts. Adding onto the problems of the cathode, the decades-long task of resolving the challenges associated to Li metal anode also remains to be solved. Together, these challenges have prevented any significant commercial application of a sulfur-based cathode. In this thesis, we look to study, explore and enhance the performance of sulfur-based cathode tested at strict conditions (sulfur areal loading of ≥4 mg cm-2 and electrolyte to sulfur ratio of ≤8 µL mg-1s). Two class of sulfur-based cathodes will be presented to bypass and resolve the problems associated with strict conditions testing, successfully achieving significantly enhanced performance. The first class will be presented in Chapter 3 and 4. Chapter 3 will be the investigation of a specific carbon material with hollow structures and a porous shell with the objective of surpassing commercial carbon material in terms of performance at first relaxed testing conditions. An emulsion-based polymerization technique was used to simultaneously create large macropores in the form of hollow structure and mesopores on the shell. Significant performance improvements were observed in terms of rate performance and cycle life. In Chapter 4, this material was tested at strict conditions through further development by employing an aerosol based agglomeration technique. We found enhanced performance at strict conditions, but the performance was still not ideal. Noting from the research trends in literature, we decided that the use of Li metal is quite detrimental to the performance of sulfur-based batteries. Therefore, our subsequent work focused on a second class of sulfur batteries, that is, the Li2S cathode. This unique configuration of a sulfur-based cathode bypasses the need of using a metallic Li metal as the Li can now be sourced at the cathode. However, Li-ion extraction (charging) from the commercially available Li2S is difficult and requires inefficient electrochemical activation. Chapter 5 and 6 will be focus on the identification and application of electrode additive to activate commercially available Li2S. These techniques have a large emphasis on ease of implementation and functionality at strict testing conditions. The first material is Li3PS4, which was found to function down to 10 wt.% in the electrode composition. The second is Na2S, which was found to function at an exceptionally low 1 wt.% in the electrode composition. Notably, both of these techniques do not require sophisticated material synthesis techniques and take readily commercially available material to achieve exceptional performance at strict conditions. Chapter 7 presents a summary of the findings in this thesis. Overall, this thesis demonstrated two different sulfur-based cathodes which can successfully function at low electrolyte to sulfur ratios and high sulfur areal loadings. A brief discussion will be given on potential future research directions of sulfur-based batteries and areas that require further improvements.