Revealing the conversion mechanisms in metal-sulfur batteries via computational methodology and in-situ spectroscopy

Abstract

Rechargeable metal-sulfur batteries are composed of a metal anode and a sulfur cathode. They present the merits of high energy and low cost, however, the practical applications are still impeded by low specific capacity, a large dosage of electrolyte, and unsatisfactory cycling stability. This is possibly due to the unclear mechanistic insights into the conversion reactions in metal-sulfur batteries. The instability of both polysulfides intermediates and metallic anode under atmosphere restricts comprehensive characterizations and deep understandings. This leads to the lack of targeted designs for metal anode and sulfur cathode, for example, the host materials for metal anode and catalysts for sulfur cathode. This thesis developed a series of in-situ spectroscopic techniques and computational methodologies to explore the conversion mechanisms in metal-sulfur batteries, which provides fundamental knowledge and practical inspirations for battery applications. First, we innovatively employed the in-situ synchrotron X-ray diffraction, in-situ Raman spectroscopy, in-situ electrochemical impedance spectra and theoretical computations to obtain better understanding of the Li nucleation/deposition processes. A design principle was suggested for Li host to overcome the electrolyte loss, that is, uneven growth of Li structure and the crack of SEI layer must be simultaneously controlled. Benefitting from the 3D lowsurface- area defective graphene host, Li metal anode achieves stable cycles (e.g., 1.0 mAh cm-2) with a low electrolyte loading (10 μL). Second, we demonstrate, for the first time, the reversible sulfur oxidation process in AlCl3/carbamide ionic liquid, where sulfur is electrochemically oxidized by AlCl4 - to form AlSCl7. The reaction pathways, AlSCl7 oxidized products, and SCl3 + intermediates are well confirmed by means of in-situ synchrotron-based analysis, high-resolution microscopic images, spectroscopic analysis, and theoretical computations. The sulfur oxidation is: 1) highly reversible with an efficiency of ~94%; and 2) workable within a wide range of high potentials. As a result, the Al−S battery based on sulfur oxidation can be cycled steadily around ~1.8 V, which is the highest operation voltage in Al−S batteries. Third, we formulate for the first time, design principles to boost electrocatalytic sulfur reduction reaction (SRR) activity by controlling the Gibbs free energy of polysulfide species in a group of 3d unary and binary transition-metal clusters. SRR reactivity trend is established through a quantitative correlation of 3d-orbital charges with Gibbs free energy and catalytic activity. The design principles and reactivity trend are 1) readily applied to boost SRR activity through adjustment of natural material property, and 2) appear universal for rational design of more-efficient catalysts. Fourth, we have proposed a general rule to boost lean-electrolyte sulfur reduction by controlling the catalyst-solvent interactions. As evidenced by synchrotron-based analysis, insitu spectroscopy and theoretical computations, the catalyst-solvent binding strength plays a crucial role in lean-electrolyte performance. Benefitting from the strong interaction between solvent molecules and cobalt catalyst, the lithium−sulfur battery achieves stable cycling with only 0.22% capacity decay per cycle under lean-electrolyte conditions. Compared to the battery with flooded electrolyte, the lean-electrolyte battery with an electrolyte/sulfur mass ratio of 4.2 maintains 79% capacity, which is the highest capacity retention among systems with lowest electrolyte dosages reported so far. Last, we demonstrate the SRR catalyst failure caused by electrophilic substitution between polysulfides and catalyst. This leads to the surface vulcanization of catalyst and more severely, the concomitant catalyst dissolution into electrolyte. Unlike other conventional electrocatalytic reactions, the failure of SRR catalyst does not depend on applied overpotentials. It is confirmed via a series of operando techniques including in-situ synchrotron X-ray diffraction, Infrared and ultraviolet–visible spectra together with theoretical computations. The proposed catalyst failure mechanism is universally extended to 3d, 4d and 5d (e.g. Co, Rh and Pt) metal catalysts.