High‑Performance Li/S Batteries Enabled by CeO₂/RGO Composite‑Coated Separators

Abstract

We report a lightweight separator modified with a two‑dimensional composite of reduced graphene oxide (RGO) anchored with cerium oxide (CeO₂) nanoparticles. The conductive RGO network facilitates rapid electron transport, while the CeO₂ surface chemically binds high‑order polysulfides (Li₂S_n, 4≤n≤8), effectively suppressing the shuttle effect. Li/S cells equipped with this composite‑coated separator deliver an initial capacity of 1,136 mAh g⁻¹ at 0.1 C and retain 75.7 % after 100 cycles, surpassing conventional separators in both capacity and Coulombic efficiency.

Background

Rechargeable lithium/sulfur (Li/S) batteries promise theoretical capacities of 1,672 mAh g⁻¹ and energy densities up to 2,600 Wh kg⁻¹, making them attractive for grid‑scale storage and electric vehicles.1,2 However, practical deployment is hindered by two key issues: (i) the intrinsic insulating nature of elemental sulfur (S₈) and its discharge products (Li₂S₂/Li₂S) limits active‑material utilization; (ii) soluble polysulfides migrate through the separator, reducing Coulombic efficiency and cycle life.3,4 Recent strategies—confining sulfur in porous conductive hosts, inserting polysulfide‑adsorbing interlayers, or optimizing electrolytes—have made progress but cannot fully eliminate the shuttle effect.5,6 Consequently, separator modification has emerged as a promising route to block polysulfide migration while maintaining ionic conductivity.

Functional coatings such as graphene, activated carbon, polypyrrole, and various metal oxides (Al₂O₃, MgO, NiFe₂O₄, SiO₂) have been explored to enhance separator performance.7–15 RGO, with its high electrical conductivity and large surface area, can lower charge‑transfer resistance (R_CT) and serve as an upper‑current collector.16 Metal oxides, in turn, bind polysulfides via strong chemical interactions, but adding a separate interlayer often increases cell mass and reduces energy density. Our approach integrates these advantages in a single, thin composite coating that preserves the lightweight nature of commercial separators.

Methods

Materials and Reagents

Graphene oxide (GO) was sourced from The Sixth Element (Changzhou). Ce(NO₃)₃·6H₂O, acrylic acid, and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai). Polyvinylidene fluoride (PVDF) and N‑methyl‑2‑pyrrolidone (NMP) came from Kynar and Tianjin Guangfu Chemical Reagent, respectively. Nanosulfur (10 wt % aqueous suspension) was obtained from Alfa Chemistry. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1,3‑dioxolane (DOL), and 1,2‑dimethoxyethane (DME) were from Sigma‑Aldrich. Super‑P, Celgard 2400 separators, aluminum foil, and lithium metal anodes were purchased from Li Zhi Yuan battery sales department. All reagents were analytical grade and used without further purification.

Preparation of CeO₂/RGO Composite and Modified Separator

CeO₂ nanoparticles were synthesized by a polymer pyrolysis route.1,2 In brief, Ce(NO₃)₃·6H₂O and acrylic acid were dissolved in 50 mL deionized water, stirred at 40 °C until a dry precursor formed, and calcined at 200 °C (2.5 h) to yield a polyacrylate salt. A subsequent calcination at 600 °C (3 h) produced CeO₂ nanoparticles.

The CeO₂/RGO composite was assembled via a simple hydrothermal process. Four grams of GO were dispersed in 40 mL water and sonicated for 1 h. 0.1 g of the CeO₂ nanoparticles were added, stirred for 2 h to promote self‑assembly, and then transferred to an autoclave. Heating at 140 °C for 4 h followed by overnight drying at 60 °C yielded the composite.

To coat the separator, 90 wt % CeO₂/RGO and 10 wt % PVDF were dispersed in NMP, ground for 40 min, and spread onto a Celgard 2400 separator (spreader height 10 mm). The coated separator was dried at 60 °C for 6 h.

Electrode Preparation and Battery Assembly

The sulfur cathode was fabricated by mixing 80 wt % sulfur composite (sulfur anchored in polypyrrole nanofiber network, see Ref. 22), 10 wt % Super‑P, and 10 wt % PVDF in NMP, then coating onto aluminum foil at a loading of ~2.0 mg cm⁻². The electrode was vacuum‑dried at 60 °C for 6 h. Coin cells (CR‑2032) were assembled with the sulfur cathode, CeO₂/RGO‑coated separator, lithium metal anode, and 1.0 M LiTFSI + 0.1 M LiNO₃ in a 1:1 (v/v) DOL/DME electrolyte (≈30 µL). Discharge/charge tests were conducted between 1.5 and 3.0 V at 0.1 C; impedance spectra were recorded from 0.01–1 MHz.

Characterization

SEM (NovaNano SEM450) and TEM (JEM2010F) examined morphology; XRD (Vinci, AXS) confirmed phase composition; XPS (ESCALAB250Xi) identified surface chemistry; Raman (LabRAM HR Evolution) assessed disorder; BET/BJH (Autosorb iQ) measured surface area and porosity; electrochemical tests employed a BTS‑5 V 20 mA system.

Results and Discussion

Powder XRD confirmed the cubic CeO₂ lattice (JCPDS 65‑2975) and the successful reduction of GO to RGO (peak shift from 11.5° to 25°). The composite’s diffraction pattern retained both CeO₂ and RGO signatures, indicating a clean, phase‑pure material.

Raman analysis showed an increase in the I_D/I_G ratio from 0.874 (RGO) to 0.915 (CeO₂/RGO), evidencing the anchoring of CeO₂ nanoparticles on the graphene sheets. The CeO₂ characteristic peak at 455 cm⁻¹ further validated the composite’s integrity.

N₂ adsorption revealed a BET surface area of 59.62 m² g⁻¹, a pore volume of 0.1331 cm³ g⁻¹, and an average pore size of 9.21 nm, providing ample pathways for electrolyte infiltration and ion transport.

SEM/TEM images showed uniform dispersion of ~10‑nm CeO₂ particles on RGO sheets, preventing aggregation and preserving the 2‑D architecture. The coated separator’s surface, with a ~15 µm composite layer, effectively blocked polysulfide diffusion while maintaining high porosity for ion flow.

Electrochemical testing demonstrated that cells with the CeO₂/RGO separator exhibited stable voltage plateaus and lower polarization (ΔE = 0.224 V) compared to cells with a pristine separator (ΔE = 0.238 V). At 0.1 C, the composite separator delivered 1,136 mAh g⁻¹ initially and 886 mAh g⁻¹ after 100 cycles (75.7 % retention), whereas the unmodified separator only achieved 992 mAh g⁻¹ and 501 mAh g⁻¹, respectively. At 1 C, the composite separator maintained 917 mAh g⁻¹ and 72.9 % capacity retention, underscoring its superior rate capability.

Nyquist plots revealed a smaller semicircle for the composite‑coated cell, indicating reduced R_CT due to the conductive RGO network and chemisorption of polysulfides by CeO₂. The steeper Warburg slope further suggested enhanced Li⁺ diffusion.

H‑type cell experiments confirmed the physical blockade of polysulfide migration: the solution on the anodic side remained clear after 16 h with the composite separator, whereas it turned dark brown with a pristine separator.

XPS after cycling showed S 2p peaks corresponding to S–O and metal‑sulfate species, and a slight shift in Ce 3d binding energies, indicating strong Ce–S interactions and confirming the chemical capture of polysulfides.

Conclusions

The polymer‑pyrolysis/hydrothermal route produced a lightweight, 2‑D CeO₂/RGO composite that, when coated onto a commercial separator, markedly improves Li/S battery performance. The composite achieves an initial capacity of 1,136 mAh g⁻¹ at 0.1 C, with 75.7 % retention after 100 cycles, and outperforms cells using unmodified separators in both capacity and Coulombic efficiency. These results validate the dual role of RGO (conductivity) and CeO₂ (polysulfide capture) in advancing Li/S technology.

Abbreviations

CeO₂

Cerium oxide

DME

1,2‑Dimethoxyethane

DOL

1,3‑Dioxolane

GO

Graphene oxide

HRTEM

High‑resolution transmission electron microscope

Li/S

Lithium/sulfur

LiTFSI

Lithium bis(trifluoromethanesulfonyl)imide

NMP

N‑methyl‑2‑pyrrolidone

PVDF

Polyvinylidene fluoride

R_CT

Charge‑transfer resistance

RGO

Reduced graphene oxide

SAED

Selected area electron diffraction

SEM

Scanning electron microscope

TEM

Transmission electron microscope

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffraction