Lithium-sulfur (Li-S) batteries are a promising energy storage device with exceptionally high specific capacities and energy densities. The widespread applications of the technology, however, have been hindered by issues such as sluggish reaction kinetics, polysulfide shuttle effect and Li dendrite growth.

This thesis begins with the study of the precipitation process in the sulfur cathode, which is identified as the limiting step to achieve a high rate capability. First, we develop a discharge model of Li-S batteries based on our experimental observations. The surface nucleation and growth kinetics coupled with electrochemical reactions is exploited for describing the Li₂S precipitation. It is predicted that promoting the growth of Li₂S particles, including lowering the initial nuclea...[ Read more ]

Lithium-sulfur (Li-S) batteries are a promising energy storage device with exceptionally high specific capacities and energy densities. The widespread applications of the technology, however, have been hindered by issues such as sluggish reaction kinetics, polysulfide shuttle effect and Li dendrite growth.

This thesis begins with the study of the precipitation process in the sulfur cathode, which is identified as the limiting step to achieve a high rate capability. First, we develop a discharge model of Li-S batteries based on our experimental observations. The surface nucleation and growth kinetics coupled with electrochemical reactions is exploited for describing the Li₂S precipitation. It is predicted that promoting the growth of Li₂S particles, including lowering the initial nucleation rate and providing a suitable amount of initial nucleation sites, can prolong the battery’s discharge capacity. Guided by the model, we further design the N-TiO₂ nanowire decorated carbon cloth electrode. The nitrogen doping in the nanowires allows for a higher electrical conductivity and stronger polysulfide binding ability. During discharge, Li₂S can be deposited along the nanowire array, instead of accumulating on the electrode surface like a resistive thin film. The design leads to a prolonged discharge capacity, 180% higher than that achieved by a pristine carbon cloth electrode at 8.0 mA cm^-2 (4.8 mg_sulfur cm^-2). Further, we demonstrate that, in Na-S batteries, the precipitation of Na₂S₂ follows a nucleation and growth process similar to Li-S. From the thermodynamic calculation and spectroscopic studies, we reveal the difference in the compositions of final solid discharge products between Li and Na-based systems.

To enhance the cyclability, we need to prevent the parasitic reactions between Li anode and solvated discharge intermediates (lithium polysulfides, LiPS). We first look at the anode/electrolyte interface and propose the self-cleaning Li-S battery by employing InI₃ as a bifunctional electrolyte additive. On one hand, In is electrodeposited onto the anode prior to Li plating during the charging process, forming a chemically and mechanically stable solid electrolyte interphase (SEI) to resist the LiPS corrosion on Li. On the other hand, by adequately overcharging the battery, the triiodide/iodide redox mediator can transform the Li₂S side products deposited on the Li anode and separator into LiPS, which can be recyclable in the cathode. The second approach is to prevent LiPS dissolution from the cathode. We design an in-situ encapsulation strategy for sulfur/carbon (S/C) composite, which could be realized by a SbF₃-based pre-coating. During the in-situ encapsulation process, liquid electrolyte can wet the S/C composite. Meanwhile, catalyzed by SbF₃, liquid electrolyte can polymerize to form a dense Li⁺-conducting solid polyether electrolyte (SPE) layer on the composite surface. Combining DFT calculation and spectroscopic studies, SbF₃ also proves to have strong anchoring effects for the LiPS, working together with the SPE layer to suppress LiPS dissolution.

The Li-S full battery further requires a reversible and safe Li-based anode. First, we design two-step spontaneous reactions for protecting the porous Li electrode. The spontaneous reaction between molten Li and sulfur-impregnated carbon nanofiber drives the formation of a porous Li electrode, comprising fibrous networks of a Li shell and a carbon core. To stabilize the Li/electrolyte interphase, we solvate BiF₃ using P₂S₅ as the dissolution promoter to form the precursor solution. By reacting with the precursor solution, Li can be coated with a tightly anchored composite layer of Li₃Bi and LiF. Besides the role of solvating BiF₃, P₂S₅ polymerizes with the residual Li₂S in the porous scaffold to form a glassy solid electrolyte layer that blocks LiPS crossover. This protected porous Li electrode enables the Li-S battery of a high loading (10.2 mg_sulfur cm^-2) at a current density (6.0 mA cm^-2) to survive for 200 cycles. Second, we adopt the Si anode to assemble a high-safe Li-ion-S full battery. In this case, the protected Li/CNF composite can act as a functional interlayer, which could be facilely inserted between the separator and Si anode during assembly. The interlayer can lithiate the Si anode, resist the polysulfide crossover and perform as the electrolyte reservoir. Such a strategy simplifies the complicated prelithiation process for the Si anode, while bringing about reliable safety and cyclability (up to 94% capacity retention for 200 cycles).

Keywords: lithium-sulfur battery; precipitation kinetics; polysulfide dissolution; lithium dendrite.