(200d) A Sustainable and Scalable Technology to Produce High Performance Mesoporous Silicon for Lithium Ion Batteries

This presentation will provide an overview of our recent efforts on developing a new technology for the sustainable and scalable manufacturing of mesoporous silicon for the application of lithium ion batteries (LIBs). Silicon has the potential as an anode material to increase lithium-ion cell capacity but the associated volume change during lithiation leads to electrode degradation and loss of capacity during cycling. While mesoporous morphologies can potentially address these issues, magnesiothermic reduction (MgTR) of silica to silicon is currently the only method for potential bulk manufacturing. However, due to a distinct lack of mechanistic understanding of this process, predictive and sustainable design of mesoporous silicon is not possible [J. Mater. Chem. A, 2018, 6, 18344].

To better understand the processes, we have systematically studied the process conditions using a model silica system [J. Mater. Chem. A, 2020, 8, 4938]. Further, we performed a complete characterisation of both the reactant and product structures in order to map how reaction conditions affect the properties of the electrode materials and provide a processing-structure-property-performance relationship. Using this knowledge, we demonstrated the design of high performance silicon using any given silica feedstock.

Next, in order to make the process scalable and sustainable, we used three strategies.

(1) We used bioinspired silica (BIS) as a sustainable, economical and scalable feedstock [PCT/GB2016/052705] and the battery performance of porous silicon structures thus produced showed that they were superior compared to other silica sources.

(2) Decreasing the MgTR reaction temperature to make it more sustainable compromises the yield due to an insufficient drive to overcome the activation energy barrier. We addressed this challenge by reducing the particle size of the feedstock to the nanoscale such that their high surface energy could help overcome the energy barrier. We obtained high yields of porous silicon at temperatures previously shown to be too low for the MgTR to even proceed. This phenomenon can be exploited in order to trigger the conversion of larger, commercial sources of silica at low temperatures to porous silicon with high yields.

(3) Scaling up MgTR has proven difficult since, at large scales, the thermal effects of the MgTR are significant and lead to sintering and loss of porosity. By carefully investigating the role of NaCl as a thermal moderator, we identified the critical factors for controlling scale-up. Trials at different scales showed that applying this knowledge of the thermal moderators, we were able to maintain product properties at larger scales.

Overall, we report the first process for the predictive design of high-performance mesoporous silicon for LIBs such that the method is scalable and sustainable. Our results have established pathways for scaling-up MgTR method such that it can now be taken forward to target specific porous silicon properties.