In-Operando FTIR Study on the Redox Behavior of Sulfurized Polyacrylonitrile As Cathode Material for Li-S Batteries

Abstract

Sulfurized polyacrylonitrile (SPAN) has been shown to be an attractive cathode material for Lithium-Sulfur (Li-S) batteries demonstrating high gravimetric capacity of up to 650mAh/g, and stable cycle performance due to the lack of a polysulfide shuttle in commercially viable carbonate electrolyte. While carbonate electrolyte works well with the cathode of Li-SPAN batteries, it is known to destabilize lithium metal, leading to inactive lithium on the anode and poor coulombic efficiencies (CE). Ether electrolytes in contrast have shown much more stable lithium metal behavior, with consistently high CE. While ether-based electrolytes offer advantages in their ability to stabilize the lithium metal anode, SPAN’s performance in ether-based electrolytes suffers from rapid capacity fade possibly due the dissolution of polysulfide species into the electrolyte during the charge discharge cycle. In this work, we investigated the redox behavior of SPAN in ether electrolytes using in-operando FTIR, and the role of lithium nitrate in cycling stability in SPAN batteries. Our cyclic voltammetry measurements have revealed that the presence of lithium nitrate in sufficient concentration (0.5M) prevents the appearance of redox peaks at 2.1V and 2.3V associated with soluble polysulfides seen in cells using ether electrolyte lacking lithium nitrate. High concentrations of lithium nitrate also prevented rapid capacity loss in cells made using ether electrolyte during long term cycling. XPS measurements showed higher concentrations of lithium fluoride in cells cycled with high concentrations of lithium nitrate, suggesting the formation of a robust cathode electrolyte interface. We developed an in-situ FTIR cell utilizing an attenuated total reflectance (ATR) accessory that can detect the major features of SPAN, such as the cyclized ring structure, the C-S bond, and the S-S bonds. For the in-operando cell, we fabricated a freestanding SPAN cathode and constructed a Li-SPAN cell on top of the ATR accessory. We examined the behavior of polysulfides to understand the role that lithium nitrate plays in preventing the shuttle effect by examining the S-S vibrational modes at 500 cm-1. Additionally, we studied the interactions between the lithium ion and the carbon backbone of SPAN by examining the cyclic behavior of vibrational modes associated with SPAN’s ring structure in the fingerprint region (1000-1500 cm-1), and ring breathing modes (800 cm-1). These peaks disappear during discharge from our in-situ spectra when the cell is at 1.7 V, which corresponds with the major redox peak of SPAN and reappears and returns to its initial position at 2.35V during charge. This indicates that the change in the ring structure is associated with a redox process, which we attribute to the intercalation of lithium into the SPAN structure.