Advanced Li2S Cathode Boosts Capacity and Stability in Solid-State Batteries

By Nidhi DhullReviewed by Laura ThomsonJul 1 2024

A recent article published in Communications Materials proposed a Li₂S-based cathode composite with a three-dimensional (3D) ion/electron-conducting structure. Mechanically reinforced with carbon fibers, the cathode exhibited a high discharge and reversible capacity.

Background

All-solid-state lithium-sulfur batteries (ASSLSBs) with solid electrolytes (SEs) exhibit high theoretical specific capacity, high energy density, non-flammability, and the natural abundance and low toxicity of sulfur, which are promising for next-generation energy storage systems.

However, the electrochemical performance of ASSLSBs is constrained by the insulating nature of sulfur and Li₂S, and severe cathode-related volumetric changes while cycling. Therefore, realizing an optimal Li₂S-based cathode composite requires reducing the cathode SE’s particle size to <1 μm, improving the mixed (ion/electron) conductivity, and constructing a stable cathode framework with 3D conductive pathways.

All three requirements cannot be accomplished concurrently using conventional mechanical ball-milling for synthesizing SEs. Thus, this study utilized a Li₂S-LiI active material (AM) solution infiltrated into a mesoporous carbon replica with ~10-nm-sized pores (CR10), liquid-phase synthesized Li₆PS₅Br SE (SE-liq), and vapor-grown carbon fibers (VGCF) to fabricate an AM-CR10/SE-liq/VGCF (ACSV) composite cathode.

Methods

The liquid-phase synthesis of Li₆PS₅Br SE (SE-liq) was carried out using Li₂S and P₂S₅ in a molar ratio of 3:1 and a stabilizer-free super-dehydrated tetrahydrofuran (THF) solvent. In addition, Li₆PS₅Br SE was fabricated by ball milling (SE-bm) using Li₂S, P₂S₅, and LiBr in a molar ratio of 5:1:2 for comparison.

Both the SEs were obtained in a pellet form, sintered at 550 °C for six hours in an argon atmosphere, and cooled to room temperature naturally.

Monodisperse 10-nm-diameter silica nanosphere colloidal crystals synthesized using a hydrothermal method were used as a template to prepare CR10. Additionally, l-arginine was selected as a base catalyst, furfuryl as a carbon source, and oxalic acid as an acid catalyst. 

Li_3.45Ge_0.45P_0.55S₄ SE (LGPS0.55), used as the separating layer of the ASSLSBs, was synthesized by vibration milling using Li₂S, P₂S₅, and GeS₂ in a molar ratio of 69:11:18. Finally, AM-CR10, SE-liq, and vapor-grown carbon fibers (VGCF) were combined in an AM-CR10/SE-liq/VGCF weight ratio of 5:4.5:0.5 to form the ACSV cathode composite.

The synthesized materials were characterized by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Their particle sizes were measured using a laser scattering particle size distribution analyzer. In addition, structural and morphological analyses were performed using field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy.

Conductivity measurements were performed on an electrochemical workstation, including alternating-current electrochemical impedance spectroscopy and direct-current polarization. Furthermore, density functional theory (DFT) calculations were performed to investigate the effects of incorporating I⁻ anions into the S²⁻ sites in the cathode.

Finally, ASSLSB cells were assembled from the fabricated components and an indium anode. Their electrochemical performances were evaluated through galvanostatic charge-discharge measurements on a multi-channel galvanostat.

Results and Discussion

The ACSV composite cathode exhibited high mixed conductivity and mechanical stability. This was attributed to the LiI activation of Li₂S, which substantially enhanced the ionic conductivity of the AM and the redox kinetics of Li₂S/S.

The close contact between the AM and CR10 ensured robustness against the cycling-induced stress. The AM sufficiently filled the porous carbon replica without damaging it.

In the DFT calculations, pristine Li₂S exhibited insulating behavior with zero density of states at the Fermi energy level and a band gap of ~3 eV. Alternatively, the Li₂S-LiI system exhibited conducting characteristics. Thus, LiI modification enhanced the AM's ionic and electronic conductivities.

The liquid-phase method proved to be time- and energy-saving compared to conventional ball milling for constructing SE-liq with high conductivity (2.22 mS/cm at 25 °C). The particle size distribution analysis of SE-liq revealed the presence of 0.1-1-µm-sized particles, considerably smaller than those of SE-bm (1-30 µm).

The micromorphological analysis revealed that SE-bm possessed coarse particles while SE-liq possessed relatively smaller particles. Thus, the nano-sized (0.1-1 µm) SE-liq exhibited superior solid-solid interfacial contact with the AM.

Consequently, the ACSV composite cathode yielded 3D ion/electron-conducting pathways, as evidenced in the FESEM images.

The cathode with high lithium storage capacity demonstrated high discharge capacity (1009 mAh/g at the 20th cycle at 0.107 mA/cm²), excellent cycling stability (retention rate of 82.8% after 100 cycles at 0.214 mA/cm²), and ultrahigh rate performance.

Conclusion

The researchers successfully fabricated a Li₂S-based cathode composite comprising a hybrid AM-CR10, a Li₆PS₅Br SE obtained by liquid-phase synthesis, and VGCFs. Overall, the cathode exhibited exceptionally high mixed conductivity and a stable structure. The synthesis method was more efficient than conventional ball milling.

The experimental results confirmed the potential of the cathode fabricated using liquid-phase methods to mitigate the insulating property of Li2S, inhibit the volumetric change within the cathode, and improve the solid-solid interfacial contact between Li₂S and SE.

Aside from these attributes, the ACSV-type multifunctional composites' enhanced three-dimensional mixed-conductivity and mechanical robustness are promising for realizing high-performance ASSLSBs.

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