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

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.

solid state batteries

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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. 

Li3.45Ge0.45P0.55S4 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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Source:

Jiang, P. et al. (2024). A composite cathode with a three-dimensional ion/electron-conducting structure for all-solid-state lithium-sulfur batteries. Communications Materials5(1), 1–14. DOI: 10.1038/s43246-024-00537-w, https://www.nature.com/articles/s43246-024-00537-w

Nidhi Dhull

Written by

Nidhi Dhull

Nidhi Dhull is a freelance scientific writer, editor, and reviewer with a PhD in Physics. Nidhi has an extensive research experience in material sciences. Her research has been mainly focused on biosensing applications of thin films. During her Ph.D., she developed a noninvasive immunosensor for cortisol hormone and a paper-based biosensor for E. coli bacteria. Her works have been published in reputed journals of publishers like Elsevier and Taylor & Francis. She has also made a significant contribution to some pending patents.  

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