Polysulfide Immobilization Through C-S Bond Formation for Long-cycle Life Li-S Batteries

Ayda Rafie

doi:10.17918/00000511

Li-S batteries are considered the next candidate for commercialization, as their theoretical gravimetric energy density reaches ~2,510 Wh/kg based on discharge voltage of ~2.15 V. The high theoretical capacity of these batteries arises from the multi-electron reactions, making them an attractive candidate to replace Li-ion batteries. Despite so many advantages and potential applications of Li-S batteries, there are challenges in achieving a high capacity and stable cycle life. First, sulfur as the active material is insulating (conductivity= ~5.0 x 10-30 S/cm). To enable sulfur utilization, it needs to be in close contact with a conductive host material. Second, sulfur undergoes a substantial volume change of ~80% in each cycle due to lithiation/de-lithiation from elemental S8 (density: 2.07 g/cm3) to the discharge product Li2S (density:1.66 g/cm3). However, the main challenge is related to the formation of soluble lithium polysulfides. Elemental sulfur used in these batteries is reduced to the highly soluble lithium polysulfide intermediates (Li2Sx, 3[less than or equal to]x[less than or equal to]8) before depositing as solid lithium sulfide (Li2S) discharge product. Although these highly soluble intermediates can enhance the reaction kinetics of the battery, they lead to the infamous phenomena of polysulfide shuttle, which is known to seriously affect Li-S battery cycling stability. In this work, we have synthesized a sulfur-rich copolymer using inverse vulcanization reaction, introduced by Pyun et al. in 2013. We have incorporated these polymers into the freestanding electrospun carbon nanofibers (CNFs). In the first part of this dissertation, we investigate the effect of the host porosity on sulfur-rich copolymers' performance in Li-S batteries. The CNFs used in our work are used as a model to understand the effect of host porosity without any interference from binders often used in slurry processing. Despite the positive effect of the C-S bond formed in SDIB active material, we hypothesize that the cathode host's porosity still plays a significant role as loose soluble -Sn- chains start to gradually emerge from. In other words, the loose lithium polysulfides are still being formed when sulfur-rich copolymers are used. This hypothesis was also tested using a sulfur-rich copolymer with SiO₂ and ester functional groups. The result of this study further confirms our hypothesis on the formation of loose lithium polysulfides, which necessitates the design of functional groups through monomers and porous structure for the host material. In the third chapter, we investigate the role of the monomer to sulfur weight ratio as a tool to manipulate the sulfur chain length in sulfur-rich copolymers. In this work, we synthesized four different copolymers with monomer wt.% of 10, 20, 50, and 70. The effect of increasing monomer wt% on the properties of copolymers is discussed using XRD, DSC, and FTIR results. Based on our results, the sulfur-rich copolymer transitions from a semi-crystalline to an amorphous copolymer with increasing the monomer to sulfur wt ratio. Moreover, as shown previously in the literature, the increase in this ratio would result in changing the sulfur chain length in these copolymers. This change is reflected on the C-S bond peak position in the FTIR spectrum of these polymers. For this reason, we designed an in operando FTIR cell to investigate the evolution of organo-lithium polysulfides and lithium polysulfides formed as the sulfur-rich copolymers (SxDIB100-x) are cycled. Owing to our cathodes' freestanding nature and the current-collector-free design of these electrodes, the in operando cell can replicate a coin cell without altering the battery components. Using the results of in situ cell, we can monitor both C-S bond evolution in ~690 cm-1 regions, and polysulfide region ~500 cm-1. Based on our results, two different behaviors in the C-S bond and polysulfide region were observed as the cell was discharged: a redshift shift in the C-S bond peak to higher wavenumbers and a blueshift in S-S vibrational frequency in the polysulfide region. Moreover, the redshift in the C-S frequency was strongly dependent on the sulfur to monomer weight ratio (x in SxDIB100-x), showing the effect of sulfur chain length on the electrochemical behavior of the sulfur-rich copolymers. Moreover, the polysulfide region analysis further confirmed that the electrochemical reaction pathways could be altered by varying the sulfur to monomer weight ratio. In the final chapter, we demonstrate the use of thiourea (TU) as an additive for ether-based electrolyte in Li-S batteries. In this study, we have introduced thiourea as a redox-active electrolyte for Li-S batteries. We believe that the redox activity of this additive originates from the oxidation of TU to form sulfur radicals. The sulfur radicals combine to form a dimer, which results in the formation of a disulfide bond. We then tested this additive using sulfur incorporated CNFs as a cathode material. Using TU additive, the SCNF cathode achieved a stable capacity of 780 mAh/g after 700 cycles. We believe that the outstanding performance of batteries with TU electrolyte originates from the dual role of this additive as a redox mediator and a shuttle inhibitor. To show the positive effect of TU in reducing the polysulfide shuttling, we used the steady-state shuttle current measurements at four different discharge states. The shuttle current measured showed a 6-fold decrease in the steady-state shuttle current when 0.2 M TU was added to the ether-based electrolyte. Moreover, using Li2S/CNF cathodes, we showed that the charge capacity of the battery using the TU additive is 1.7 times higher than the reference cell. Based on our results, we believe that TU additive in the ether-based electrolyte can have two simultaneous effects on the performance of Li-S batteries. On one hand, it can reduce the soluble the infamous polysulfide shuttling by binding them with the C-S bond formed. On the other hand, it can be used as a redox mediator to improve the Li-S battery's reaction kinetics, i.e., facilitate re-utilization of Li2S.

Polysulfide Immobilization Through C-S Bond Formation for Long-cycle Life Li-S Batteries

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Polysulfide Immobilization Through C-S Bond Formation for Long-cycle Life Li-S Batteries

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