(352f) Mechanistic Elucidation and Electrochemical Performance of Amorphous Carbon Interfaced Graphite-Silicon Advanced Anodes

Graphite, inspite of being the mainstay in the lithium-ion battery (LIBs) industry for decades, has come to realize its own limitation of theoretical anode capacity (372 mAh g^−1).¹ Due to this reason, there has been a large momentum towards fabrication or invention of new anode materials with improved specific capacity and stable cycle performance. Alloys are a possible answer to the situation and have received much attention. Sn, Ge or Si alloys have higher specific capacity as their atomic framework aid the reaction as compared to the intercalation materials. Amongst all alloys types, silicon has garnered interest as an anode material for the next generation LIBs, owing to its high capacity, however achieving long stable cycle life is a significant challenge due to fatal and critical technical issues (e.g. volume expansion, excess solid electrolyte interface generation, particle fragmentation and pulverization, etc.) and hence impeding the practical use of Si anodes.² For commercialization, various approaches have been tried to overcome the issues like fabricating nanostructured silicon, coating or dispersing Si-nanoparticles (Si-NPs) with active or less active materials for stress reduction, generated due to volume expansion. Though these materials with better electrochemistry are quite promising, there are still several problems that need to be addressed like low tap density, uneconomical and complex processing, accelerated side reactions due to the large surface area and poor scalability.³

To address these challenges, we engineered graphite-silicon composite bridged using wheat flour-derived amorphous carbon (GCSi) for realizing high performance LIBs.⁴ GCSi composite utilizes the carbon from the wheat source to effectively confine the volume expansion and improve the electrical conductivity by developing an interface between the graphite and silicon electrode. The synthesis process uses an easily scalable pyrolysis technique in an inert atmosphere to homogenize Si-NPs, wheat flour and graphite via ball-milling. Cross sectional scanning electron micrographs of lithiated and delithiated electrodes revealed the volume changes. Compared to graphite-Si control sample, GCSi had lower volume changes suggesting accommodation of the volume expansion of Si-NPs. STEM and HAADF studies confirmed that the wheat flour derived carbon in the GCSi composite physically bridges the graphite and Si NPs. Mechanistic elucidation of the tailored composite was achieved with in-situ HR-ETEM as a function of temperature. The mixture was deposited on a MEMS ETEM heating chip, and an area was selected where graphite, wheat flour, and silicon overlapped, with the Si-NPs buried in a cluster of flour. The cluster sat on top of a sheet of graphite - all suspended over vacuum for high resolution imaging. Temperatures were ramped up from 40°C to 500°C at 12 °C min^-1 with images captured every 5 minutes. This ramp and imaging allow the capture of the material transition in-situ. The carbonization and dehydrogenation of the starch was observed at 400 °C and completed at 500 °C. As the starch homogenized, it appeared to "fuse" into the graphite. The inner defects could be seen to transition into a uniform amorphous carbon mass because of conversion of carbohydrate mass to carbonaceous entity. The BF images show the cross section of the sample as the process occurred, showing the inner structure homogenizing. The Secondary Electron recording showed as the surface of the starch goes from highly course to a smooth continuous surface. EDX was used to confirm the elemental composition of the particles during the transition. This network generated by the wheat carbon, encapsulating the Si-NPs, lead to the formation of a conductive and physical cage that results in an improved and stable performance of the Si-based anodes.

The designed GCSi composite architecture comprising about 25 wt% silicon showed a high initial discharge capacity of >1000 mAh g^−1 with high coulombic efficiency. GCSi composite anode also demonstrated exceptional rate capabilities. This stability and high performance are a result of the improved mechanical stability and improved electrochemical kinetics made possible by well interconnected composite architecture of the wheat derived amorphous carbon. Thus, the proposed strategy of incorporating an amorphous carbon network into the G-Si composites decreases the inherent issues associated with the pristine Si electrodes and provide an alternative solution for the development of advance Si-based anodes. The mechanistic elucidation of interfacial development of amorphous carbon bridging standard graphite and silicon as a function of temperature yielding advanced anode architecture will be discussed in great details using HRTEM and in-situ videos.

References:

(1) Goriparti, S.; Miele, E.; De Angelis, F.; Di Fabrizio, E.; Proietti Zaccaria, R.; Capiglia, C. Review on Recent Progress of Nanostructured Anode Materials for Li-Ion Batteries. J. Power Sources 2014, 257, 421–443.

(2) McDowell, M. T.; Lee, S. W.; Nix, W. D.; Cui, Y. 25th Anniversary Article: Understanding the Lithiation of Silicon and Other Alloying Anodes for Lithium-Ion Batteries. Adv. Mater. 2013, 25 (36), 4966–4985.

(3) Tang, J.; Dysart, A. D.; Kim, D. H.; Saraswat, R.; Shaver, G. M.; Pol, V. G. Fabrication of Carbon/Silicon Composite as Lithium-Ion Anode with Enhanced Cycling Stability. Electrochim. Acta 2017, 247, 626–633.

(4) Parekh, M. H.; Parikh, V. P.; Kim, P. J.; Misra, S.; Qi, Z.; Wang, H.; Pol, V. G. Encapsulation and Networking of Silicon Nanoparticles Using Amorphous Carbon and Graphite for High Performance Li-Ion Batteries. Carbon N. Y. 2019, 148, 36–43.