Investigating Advanced Cathode Materials for Li/Na-S Batteries Experimentally and Theoretically

Abstract

Lithium/sodium-sulfur (Li/Na-S) batteries hold practical promise for next-generation batteries because of high energy density and low cost. Development is impeded at present however because of unsatisfied discharge capacity and stability in long cycling. Advanced materials can serve as sulfur host materials to improve the capacities and stability of the lithium/sodium-sulfur batteries. More importantly, they provide suitable models with which to connect and test experimental results with theoretical predictions. This is crucial to develop insight into the relationship between electrochemical behavior of sulfur and the structural properties of sulfur host materials. This thesis explores sulfur and its intermediates adsorption/redox conversion mechanisms and investigate crucial structural-property relationships of the advanced nanomaterials as sulfur host materials in high-performance lithium/sodium-sulfur batteries. First, A unique three-dimensional hybrid of nickel sulfide and carbon hollow spheres was synthesized as a sulfur host. The uniformly distributed nickel sulfide can greatly promote adsorption capability towards polysulfides. Meanwhile, the hollow carbon spheres increase sulfur loading as well as the overall conductivity of the sulfur host. Utilized in an electrode, this 3D hybrid sulfur host achieved a capacity of 695 mA h g-1 after 300 cycles at 0.5 C and a low capacity decay of 0.013% per cycle. Second, a two-dimensional (2D) MoN-VN heterostructure is investigated as a model sulfur host. The 2D heterostructure can regulate polysulfides and improve sulfur utilization efficiency. This resulted in superior rating and cycling performance. More importantly, incorporation of V in the heterostructure can effectively tailor the electronic structure of MoN. This leads to enhanced polysulfides adsorption. Last, a two-dimensional (2D) metal-framework (MOF) is investigated as a model sulfur host for Na-S batteries. The MOF can enhance polysulfides adsorption and conversion kinetics. This resulted in superior rating and cycling performance. Through a combination of advanced experimental characterization techniques and theoretical computations based on the 2D nanomaterials, an in-depth understanding of sulfur redox and the structure-properties relationships in metal-sulfur batteries have been obtained.