Probing the effect of polymer coating, carbon material and molecule engineering of redox organic molecules for durable lithium-sulfur batteries
Keywords: Chemistry.

Lithium-sulfur (Li-S) batteries charge by oxidizing solid lithium sulfide (Li2S) into sulfur (S8) through soluble lithium polysulfide intermediates (Li2Sx), enabling a high energy density of 2500 Wh kg-1, a five-fold increase compared to traditional Lithium ion Batteries (LiBs). Such exceptionally high energy density is enabled by the reversible reaction between sulfur and lithium sulfide (Li2S) via a series of lithium polysulfides intermediates (LiPSs, Li2Sn, 2≤n≤8). However, significant challenges remain in order to build practical Li-S batteries, which are primarily attributed to the dissolution of intermediate species (LiPSs) in the electrolytes as well as the insulating nature of both sulfur and Li2S. strategies to improve durability of Li-S batteries are key to their successful application in commercial batteries. In this thesis, polymer coating, electrolyte additive, and carbon materials were used to address issues of Li-S batteries. The first portion of my research describes an efficient design of hybrid electrode structures using a solution-processable isoindigo-based polymer incorporating polar substituents. It provides the following critical features: (1) the conjugated backbone provides good conductivity; (2) functional pyridine groups provide high affinity to polysulfide species; and (3) it possesses high solubility in organic solvents. These lead to effective coating on various carbonaceous substrates to provide highly stable sulfur electrodes. Importantly, the electrodes exhibit good capacity retention (80% over 300 cycles) at sulfur mass loading of 3.2 mg/cm2, which significantly surpasses the performance of previously reported polymer-enabled sulfur cathodes. However, a challenge that was not solved in the first part of my research is the insulating nature of both sulfur and Li2S. For examples, when charging a Li2S electrode, a significant portion of each particle is electrically isolated and can be oxidized at the localized interface between the electrode/electrolyte with sufficient charge transfer. Therefore, the Li2S exhibits a large overpotential and a limited reversible capacity that is substantially lower than the theoretical value. Hence, in the second portion of my research, we employ the redox chemistry of a quinone derivative to realize efficient, fast, and stable operation of Li-S batteries using Li2S microparticles. When adding a quinone derivative with tailored properties (e.g. oxidation potential, solubility, and electrochemical stability in the electrolyte) to an electrolyte as a redox mediator, initial charging of Li2S electrodes occurs below 2.5 V at a 0.5C rate, and the subsequent discharge capacity is as high as 1300 mAh g-1. Moreover, deposition of dead Li2S is effectively prevented with the addition of the redox mediator, thus avoiding the primary cause of increasing polarization and decreasing reversible capacity of Li-S batteries upon cycling. Another primary approach to solve the above issues is to infiltrate sulfur into nanostructured conductors, such as porous carbon materials, to realize sufficient conductivity and cycling rate performance. However, most carbon host materials were tested with high electrolyte to sulfur ratios (E/S) (generally > 15 μL/mg), which compromises the cell-level energy density. Hence, it is important to re-visit the carbon structures with desired properties to enable low E/S ratio. The third portion of my thesis is to design a flower-shaped porous carbon structure that has several advantages: (1) the material has superior high surface area (> 3300 m2/g) ; (2) the pore size is less than 5 nm, which is more suitable for low E/S ratio; (3) Incorporating Nickel nanoparticles onto the carbon flower gives stronger binding interaction with LiPSs and better reaction kinetics. Through the desired properties of carbon flower as host material for sulfur, we successfully demonstrated that higher capacity and higher cycle retention were enabled by using the carbon flower-sulfur (CF-S) electrode in low E/S ratio (< 5 μL/mg)