Polypyrrole/ZnO Interlayer Boosts Lithium/Sulfur Battery Performance with Enhanced Polysulfide Suppression

Abstract

A novel interlayer comprising polypyrrole (PPy) nanofibers decorated with zinc oxide (ZnO) nanoparticles was engineered by coating a PPy/ZnO slurry onto a Celgard 2300 separator. The three‑dimensional hierarchical network efficiently traps soluble polysulfides, mitigating the shuttle effect and enhancing conductivity. A Li/S cell incorporating this interlayer delivered a stable 579 mAh g⁻¹ after 100 cycles at 0.2 C, demonstrating a clear pathway toward commercial Li/S batteries.

Background

Portable electronics and the environmental burden of fossil fuels drive the demand for lightweight, high‑energy-density storage. Lithium/sulfur (Li/S) batteries, offering 2600 Wh kg⁻¹ energy density and a theoretical capacity of 1672 mAh g⁻¹, are a prime candidate. However, poor electronic conductivity of sulfur and the polysulfide shuttle limit their practical use. Traditional approaches—nanostructured encapsulation, electrolyte engineering, binder modification—have shown limited success. Interlayers placed between cathode and separator can trap polysulfides, but carbon‑based materials often lack sufficient adsorption capacity. Conductive polymers such as proton‑doped PPy can bind polysulfides via H‑bonding, while polar metal oxides like ZnO can form covalent interactions with polysulfides, albeit at the cost of conductivity. Integrating PPy with ZnO offers a balanced solution: the PPy network provides electrical pathways and physical barriers, whereas ZnO enhances chemical trapping without compromising conductivity.

Methods

Preparation of PPy/ZnO Interlayer

PPy nanofibers were synthesized following established protocols. 0.2 g of PPy was dispersed in 30 mL of 4 mM Zn(CH₃COO)₂•2H₂O methanol, followed by 10 mL of 0.3 M KOH methanol. The mixture was stirred at 60 °C, then centrifuged to yield PPy/ZnO composite. A slurry (80 % PPy/ZnO, 10 % Ketjen Black, 10 % PVDF) was uniformly coated onto Celgard 2300, forming the interlayer.

Preparation of S‑Cathode

Sulfur (Sigma‑Aldrich, ~100 mesh) was mixed with graphene (2:1 w/w) and heated to 155 °C under argon for 12 h. The resulting composite, mixed with Ketjen Black and PVDF (80:10:10), was cast on carbon‑coated Al foil, dried at 60 °C, and punched into 14 mm disks, achieving ~1.3 mg cm⁻² sulfur loading.

Material Characterization

FE‑SEM (Leo‑1530), TEM (JEM‑2100F), XRD (Smart Lab), FTIR (TENSOR 27), and XPS (Thermo ESCALAB 250Xi) were employed to analyze morphology, structure, and chemical states.

Electrochemical Testing

Half‑cells were assembled in an Ar glovebox (99.9995 % purity). Li foil served as anode; electrolyte consisted of 1 M LiTFSI + 0.1 M LiNO₃ in a 1:1 DOL/DME mixture (~30 µL). Tests were performed between 1.7–2.8 V using a Neware battery station; CV (0.1 mV s⁻¹) and EIS (10⁻²–10⁵ Hz) were recorded with a VersaSTAT 4.

Results and Discussion

The PPy/ZnO composite displayed a 3‑D hierarchical network of ~80 nm PPy fibers decorated with ~15 nm ZnO nanoparticles, confirmed by SEM/TEM and XRD (hexagonal wurtzite). FTIR revealed characteristic PPy peaks and a Zn–O stretch at 437 cm⁻¹. The interlayer thickness was ~12 nm.

Electrochemical data showed that the PPy/ZnO interlayer reduced polarization, as evidenced by sharper CV peaks and a lower ΔE in charge/discharge curves. The initial discharge capacity was 1194 mAh g⁻¹, falling to 579 mAh g⁻¹ after 100 cycles at 0.2 C—significantly higher than the 318 mAh g⁻¹ of cells without the interlayer.

Rate performance improved markedly: at 0.2–2 C the interlayer cell retained 404–951 mAh g⁻¹, while the control dropped to 144 mAh g⁻¹. Reversible capacity recovered upon returning to lower rates, indicating robust polysulfide confinement.

EIS measurements revealed a drop in charge‑transfer resistance from 66.3 Ω to 35.9 Ω after interlayer insertion, and further to 12 Ω after 50 cycles, confirming enhanced electronic pathways and reduced polarization. Warburg impedance remained low, supporting efficient ion diffusion.

XPS analysis post‑cycling confirmed strong interactions: C–N/C–S, C–O, and N functionalities in PPy; ZnO peaks at 1022.3/1045.1 eV; and sulfur signals indicative of thiosulfate formation, demonstrating chemical trapping of polysulfides.

Conclusions

Embedding a PPy/ZnO nanocomposite interlayer on the separator effectively suppresses polysulfide shuttling, enhances conductivity, and extends cycle life of Li/S batteries. The synergy between the conductive PPy network, nitrogen functionalities, and polar ZnO nanoparticles offers a practical route to high‑performance, commercially viable Li/S cells.