Sulfurized Polyacrylonitrile (SPAN): Mechanical Properties and Reduction Mechanisms of Carbonate-Based Solvents on Its Surface
Authors: Perez-Beltran Saul , Balbuena Perla B. 
Published: February 2020

The lithium-sulfur (Li-S) system promises the most for the next generation of rechargeable batteries, as the Li-S battery offers a high theoretical energy density of 2567 Wh/kg and raw sulfur enjoys of widespread availability at low cost. The massive commercialization of this technology, however, relies heavily on tackling yet unsolved operational issues that dramatically diminish the practical energy density delivered by Li-S batteries, such as (i) low electric conductivity of sulfur and the sulfur reduction products, (ii) continuous loss of active sulfur due to shuttling of long-chain lithium polysulfides, and (iii) dendritic growth on the anode surface.

An alternative strategy to solve these operational issues is the chemical sequestration of sulfur through sulfur-carbon bonding, which has shown improved electrochemical stability and longer cycling stability compared to the commonly used strategy of physical confinement of sulfur into a carbonaceous porous matrix. The sulfur polyacrylonitrile (SPAN) is now one of the most promising materials for chemical sequestration of sulfur. Experiments and theoretical calculations have shown that this material avoids the formation of long-chain lithium polysulfides and allows the introduction of carbonate-based solvents into the electrolyte, which opens up the possibility of using the same electrolyte manufacturing technology of Li-ion batteries for Li-S batteries.

Even though the advantages that SPAN materials bring for the development of the Li-S battery technology, there is still a lack of understanding on how to increase the sulfur loading, fundamental to guarantee high energy density, and also on why the SPAN performs better with carbonate-based electrolytes compared to the ether-based electrolytes traditionally used in Li-S batteries.

In this work, we aimed at characterizing the mechanical properties of the SPAN and the chemical reactions at the electrolyte-SPAN interface. Reactive molecular dynamics (ReaxFF) calculations allowed evaluating the stress-strain behavior of the SPAN and the lithium self-diffusion coefficient as a function of sulfur content and lithiation stage, whereas Ab initio molecular dynamics (AIMD) calculations aimed at identifying the reduction mechanisms of carbonate-based solvents such as ethylene carbonate (EC) and fluoroethylene carbonate (FEC) on SPAN surfaces various degrees of lithiation. We elucidated the impact that sulfur loading has on the SPAN structural stability and the accessibility of lithium into those regions where sulfur agglomerates in the SPAN structure, allowing us to find out a trade-off between sulfur loading, structural stability, and electrochemical activity. Regarding the solvent/SPAN interface dynamics, we elucidate the EC/FEC decomposition mechanisms on the SPAN surface and its dependence on the lithiation degree. We believe that our results will contribute to the development of SPAN-based cathodes with higher areal-loading and a high-quality solid electrolyte interphase (SEI) protecting the electrode.