Non-Confinement Approach Towards Employing Carbonate-Based Electrolytes in Li-S Battery

Abstract

Rechargeable lithium-sulfur (Li-S) batteries have received increasing attention due to the high theoretical capacity of sulfur cathodes (1675 mAh.g-1), abundance, environmental friendliness, low toxicity and low cost of elemental sulfur. The commercialization of this technology, however, relies heavily on tackling yet unsolved operational issues that hinder the practical energy density delivered by Li-S batteries, such as (i) low electrical conductivity of sulfur and lithium sulfide (Li2S), (ii) severe capacity fade due to shuttling of long-chain lithium polysulfides, and (iii) lithium dendrite growth. In addition, another major obstacle towards commercialization is use of ether-based electrolytes owing towards its high flammability slightly above room temperature.Carbonate-based electrolytes demonstrate safe and stable electrochemical performance in lithium ion batteries. Literature reports have demonstrated that sulfur can trigger substantial electrolyte decomposition due to formation of polysulfides to exhibit sudden battery failure. However, few types of cathodes with confinement of sulfur and cross-linking with polymer can be employed. However, higher sulfur loading, and improved conductivity of cathode still remains a prominent challenge in carbonate-based electrolyte. In this work, we have developed free-standing carbon nanofiber/S cathodes for achieving long-term cycling in Li-S batteries. The as-prepared CNF/S (1 – 5 mg.cm-2, ~50 wt% S) cathodes were used directly in 2032 type coin cells. The electrochemical tests were performed in EC:DEC based electrolyte against Li/Li+anode. The cyclic voltammetry and charge discharge results showed a single potential for conversion of sulfur to Li2S. The CNF/S cathodes delivered capacity of ~1300 mAh.g-1 during the first cycle with ~93% coulombic efficiency. The capacity of Li-S cells was stabilized at ~800 mAh.g-1 after Ist cycle and sustained up to 4000 cycles with only ~.01% decay rate per cycle. The postmortem XRD results confirmed the conversion of sulfur to Li2S and vice versa. Further, postmortem XPS analysis was conducted to understand the underlaying conversion mechanism. The preliminary results showed that the observed single plateau and long-term cycling of Li-S cells were the convoluted effect of deposited sulfur and digression from typical polysulfide formation route.