title: Carbon-Rich SiCN/SiOC Ceramics as Sulfur Cathode Supports in Lithium Sulfur Batteries
Creators: name: Qu, Fangmu
Publisher: name: UNSPECIFIED
Year: 2024

description: As one of the promising candidates for future energy storage, lithium sulfur batteries have attracted much attention due to their superior energy density and low production cost as compared with that of conventional Li ion batteries. However, the obstacles such as insulated nature, volume expansion and “shuttle effect” significantly hamper the practical commercial application of lithium sulfur batteries. Designing and building novel sulfur cathodes with conductive micro-nanostructure is one of the potential ways to solve the issues of lithium sulfur batteries. C-rich ceramic matrices provide an ideal conductive skeleton for the sulfur host due to their electronic conductivity and robust, stress accommodating mechanical properties. This dissertation presents facile and cost-effective methods for the preparation of C-rich silicon carbonitride (SiCN) ceramics, Silicon oxycarbide (SiOC) ceramics and SiCN-boron nitride (SiCN-BN) composites to be utilized as sulfur host materials for the cathode in lithium sulfur batteries to enhance their electrochemical performance.

First of all, porous C-rich SiCN ceramic matrices were synthesized by crosslinking of silicon-based polymers and subsequent pyrolysis in a temperature range between 1000 ℃ and 1600 ℃ under argon. Through the melting-diffusion technique at 155 ℃, sulfur was successfully embedded in the obtained porous ceramic matrices. After the characterization and performance testing, the impact of the initial porosity and elemental composition of the SiCN ceramics on the electrochemical performance of SiCN-S composites is addressed. It is shown that the material pyrolyzed at 1000 °C reveals a mesoporous character in line with the presence of a free carbon phase dispersed in the SiCN matrix and demonstrates the best electrochemical stability and the highest capacity (more than 310 mAh/g over 40 cycles) at a high sulfur content of 66 wt.%.

To investigate the electrochemical performance of the C-rich SiCN ceramics pyrolyzed at a relatively low temperature window compared with the first work, sulfur-containing C-rich SiCN composites were processed from SiCN ceramics which were synthesized at temperatures between 800 ℃ and 1100 ℃. The obtained results reveal that a combination of the mesoporous character of SiCN and the presence of a disordered free carbon phase makes the electrochemical performance of the SiCN matrix obtained at 900 °C superior to that of SiCN synthesized at lower and higher temperatures. A capacity value of more than 195 mAh/g over 50 cycles at a high sulfur content of 66 wt.% is achieved.

Then, we further studied the electrochemical performance of three distinct sulfur/PDC cathodes. In two of these cathodes, sulfur is incorporated into PDC aerogels based on the SiOC and SiCN systems, utilizing CO2 supercritical drying. In the third cathode, sulfur is confined within a mesoporous SiOC produced through the "polymeric spacer" method. The composite cathodes underwent electrochemical characterization, and their performances were analyzed and correlated with the chemical composition and microstructure of the obtained PDC scaffold.

In order to achieve further enhancement of the electrochemical performance of C-rich SiCN ceramic used as sulfur hosts, boron nitride was confined in the SiCN matrix. The SiCN-BN composites were synthesized via annealing mixtures comprised of C-rich SiCN ceramic powder, boric acid and urea at different temperatures (950 °C, 1100 °C, and 1250 °C). The sample SiCN-BN-950/S especially presents 445 mAh/g of the reversible capacity and 62 % of the capacity retention after 60 cycles under 3.5 ~ 3.8 mg/cm2 of areal density and 66 wt.% of sulfur loading. The excellent cycling stability is attributed to the remarkable synergistic effect of BN and C-rich SiCN ceramic matrix.

The obtained results demonstrate that C-rich SiCN/SiOC ceramics are indeed promising hosts for sulfur as the cathode in lithium sulfur batteries for enhancing their electrochemical performance. Besides, this work also presents facile, cost controllable and efficient synthesis routes for sulfur cathodes in LSB which is suitable for large-scale industrial production, as well as a reference for the utilization of ceramic materials in energy storage systems.