Sulfurized Polyacrylonitrile for High-Performance Lithium–Sulfur Batteries: In-Depth Computational Approach Revealing Multiple Sulfur’s Reduction Pathways and Hidden Li⁺ Storage Mechanisms for Extra Discharge Capacity

Abstract

Like no other sulfur host material, polyacrylonitrile-derived sulfurized carbon (SPAN) promises improved electrochemical performance for lithium–sulfur batteries, based on its compatibility with carbonate solvents and ability to prevent self-discharge and shuttle effect. However, a complete understanding of the SPAN’s lithiation mechanism is still missing because its structural features vary widely with synthesis conditions, and its electrochemical performance deviates from elemental sulfur. This study continues our research on the elucidation of the SPAN’s structural characteristics and lithiation mechanisms via computational approaches. Our models reproduce most experimental data regarding carbon’s graphitization level and conjugated ordering, sulfur–carbon covalent bonding, sulfur loading, and elemental composition. Our simulations emulate the discharge voltage observed in experiments for the first discharge, which reveals that sulfur follows multiple reduction pathways based on its interaction with the carbon backbone. Sulfur reduction takes place above 1.0 V vs Li/Li+ mostly in the SPAN-like material, with no long-chain lithium polysulfide formation. Below 1.0 V vs Li/Li+, the backbone’s electrochemical activity occurs via multiple C–Li and N–Li interactions, mostly with edge carbon atoms and pyridinic nitrogen. Moreover, we identify Li+ binding sites throughout the graphitized backbone that might lead to prohibited energy costs for Li+ deintercalation, which may explain the irreversible capacity loss between the first and second discharges. This work improves understanding of lithiation mechanisms in sulfurized carbon, which is useful for rationally designing SPAN synthesis pathways tailored to increase sulfur loading and enhanced electrochemical performance.

Sulfurized Polyacrylonitrile (SPAN): Changes in Mechanical Properties during Electrochemical Lithiation

“…For a 2.0 Li/S ratio, the volume change is close to 56%, and the density reduces from 2.26 g/cm 3 (before lithiation) to 1.67 g/cm 3 . These results are in qualitative agreement with earlier DFT calculations on the SPAN lithiation that indicate a 45.4% volume change after a lithiation close to a 2.0 Li/S molar ratio, equivalent to a discharge down to 1.0 vs Li/Li + . The cathode’s volume change measurements performed on a SPAN-based Li–S battery with a 32 wt % sulfur loading also show qualitative agreement with our results as they indicate a 22% volume increase after discharge to 1.0 V vs Li/Li + .…”

“…With regard to the density’s change, the 1.67 g/cm 3 density is in close agreement with the reported for Li 2 S (1.66 g/cm 3 ). , The density plot also shows no significant density changes with further lithium loading (above 2.0 Li/S molar ratio), dropping only to 1.6 g/cm 3 after reaching a 2.9 Li/S molar ratio. This behavior indicates that sulfur reduction completes after achieving a 2.0 Li/S molar ratio, in agreement with our earlier DFT calculations for the SPAN material . The PRDF profile for the S–S interactions and the sulfur-chain distribution plots vs the Li/S ratio in Figure a,b complement this affirmation.…”

“…Figure S4 summarizes the PRDF profiles for the S–S, S–Li, and C–Li interactions and the sulfur-chain distribution plots for model G; we observe a lithiation behavior similar to model A. Moreover, the C–Li profile suggests the C–Li bonding interactions become increasingly important with a 2.0 and above Li/S molar ratio, in agreement with our earlier DFT-based observations on the SPAN lithiation mechanism that highlight the carbonized skeleton’s electrochemical active role at lower than 1.0 V vs Li/Li + discharge voltages …”

“…The SPAN material’s complex structural features induce a single plateau and a slightly sloped voltage discharge profile centered at 1.7 V vs Li/Li + , , deviated from the typical two-sloped discharge profile found in elemental sulfur . Details on the SPAN electrochemical lithiation mechanism and its compatibility with ether and carbonate-based electrolytes are available in earlier publications on this topic from our work and that from other authors. ,,, …”

“…The SPAN structures’ lithiation proceeds similarly to the preceding sulfurization step but with lithium atoms’ insertion until the system reaches an ∼2.9 Li/S molar ratio. Multiple experimental and theoretical works conclude that SPAN lithiation proceeds beyond the theoretical 2.0 Li/S molar ratio for lithiation of elemental sulfur. ,,, Inserting lithium as a neutral atom keeps the unit cell neutral and accounts for the lithium ion coming from the anode and the electron coming from the external circuit. The ReaxFF method uses the electronegativity equalization method (EEM) to equilibrate charges every time step of the simulation automatically. , …”