Lithium-sulfur (Li-S) batteries have been considered as an attractive alternative to current Li-ion batteries due to their large theoretical capacity (1672 mA-h/g) and theoretical energy density (2600 Wh/kg) while having a low cost, an abundance of the material, and relatively non-toxic properties. However, the low cyclability and significant capacity fading during the first several cycles prevent Li-S rechargeable batteries from being commercialized. During discharge, elemental sulfur is reduced to the final product Li2S through a series of soluble intermediate species, lithium polysulfides (Li2S x, 2 ≤ x ≤ 8). Lithium polysulfides dissolved into the electrolyte in the separator can no longer participate in redox reductions, resulting in a loss of active materials, as well as a “shuttling effect” that causes capacity fading and low coulombic efficiency. Despite the fact that decades of research have attempted to solve this, the problem is still not resolved due to a lack of fundamental understanding of the system. This includes how lithium polysulfides are produced during discharge interactions with other components in the cell and the reaction mechanisms (the electrochemical and chemical processes) during cycling. The objective of this dissertation is to provide a fundamental understanding of lithium polysulfides produced during discharge of a Li-S cell. This is an essential piece of knowledge when designing and identifying the issues associated with Li-S batteries.
To begin, the morphology, thermal properties, and ionic conductivity of an ether-based nanostructured block copolymer containing lithium polysulfides were investigated. Previous work has shown that nanostructured block copolymer electrolytes containing an ion-conducting block and modulus-strengthening block has the potential of enabling solid-state lithium metal rechargeable batteries. This is of particular interest for a lithium-sulfur battery to fully explore its high energy density and capacity. Understanding the thermal and electrochemical properties of these block copolymer electrolytes containing lithium polysulfides is essential for evaluating their potential use in Li-S batteries. A systematic study of polystyrene-b-poly(ethylene oxide) (SEO) block copolymer mixed with Li2Sx with an average x value of 4 and 8 was conducted. Small angle X-ray scattering, differential scanning calorimetry, and ac impedance spectroscopy were used to measure the morphology, thermal properties, and ionic conductivities of all samples. The ionic conductivity of SEO/Li2Sx mixtures were compared with those of poly(ethylene oxide) (PEO) mixed with Li2Sx to quantify the effect of nanostructuring on ion transport. The conductivities of both SEO and PEO samples containing polysulfides with a longer average chain length higher than the same polymer containing polysulfides with a shorter average chain length at all salt concentrations, indicating that dissociation of long-chain polysulfides occurs more readily than short-chain polysulfides. Normalized conductivity was used to quantify the effect of morphology on ion transport. The results showed that SEO suppressed the migration of polysulfides relative to PEO. However, this suppression is inadequate for practical applications. In other words, cathode architectures that prevent polysulfides from entering the electrolyte are necessary for enabling Li-S batteries with block copolymer electrolytes. Nevertheless, the results obtained in this study are important as they enable quantification of polysulfide migration in Li-S batteries with imperfect polysulfide encapsulation, a limitation that applies to all known Li-S batteries.
Next, UV-vis spectroscopy with radiation wavelength in the range 200 - 800 nm was used to study different polysulfides in ether. Ex-situ UV-vis spectra were measured for chemically synthesized lithium polysulfides in TEGDME, Li2 Sx_mix | TEGDME solutions for xmix values of 4, 6, 8, and 10 and sulfur concentrations of 10, 50, and 100 mM. The peaks are generally more resolved at lower concentrations than at higher concentrations for all xmix values, suggesting a concentration dependence of spectra shape. The peak at 617 nm was used to confirm the existence of S3 •- radical anion, which supports the argument that polysulfide radical anions are stable in ether-based electrolytes, and may play an important role in Li-S reaction mechanism. Using in-situ UV-vis method was discussed and challenges for Li-S reaction mechanism study were evaluated. A new fluorinated-ether based electrolyte was explored. Its low polysulfide solubility makes it a good candidate to be used in in-situ Li-S reaction studies because UV-vis radiations do not have a large penetration path through high concentration of polysulfide-containing materials. However, the main challenge in using UV-vis spectroscopy to study Li-S reaction mechanism is the ambiguity in peak assignments arised both from a lack of spectra standards for different polysulfides. It is difficult to experimentally obtain polysulfide spectra standards because polysulfides cannot be separated. (Abstract shortened by ProQuest.)
|Advisor:||Balsara, Nitash, Minor, Andrew|
|Commitee:||McCloskey, Bryan, Persson, Kristin|
|School:||University of California, Berkeley|
|Department:||Materials Science & Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 80/03(E), Dissertation Abstracts International|
|Keywords:||Block copolymer, Lithium sulfur, Operando xas, Polysulfides, Reaction mechanism|
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