(166ac) Morphological Influence of Thermal Treatment of Barley Straw Derived Si/C Composite for Potential Application in Energy Storage

Silicon (Si) is an abundant, cheap and environmentally friendly element, which is a potential replacement for graphite as the anode material in Li-ion batteries. This is due to the high theoretical capacity of silicon (3589 mAh g^-1) which is an order of magnitude greater than that of graphite (372 mAh g^-1). However, application of bare silicon as an anode material suffers from material pulverization during Li-ion cell operation. One of the proposed strategies to mitigate this problems is to coat the silicon with carbon [1,2]

The carbon material used in this study was extracted from bio-waste, namely barley straw. The barley straw was mechanically grinded to produce powder. The powder was then stirred in 37 wt% HCl for 3 hours. The mixture underwent Buchner filtration, subsequently the acid was removed from the filtrate by rotary evaporation. The remaining solid mass was oven dried over night. The treated barley powder and silicon nano particles were wet mixed in ion-exchanged water to produce a Si/C composite. The water was later removed from the mixture by heating it in a petri dish on a hotplate. The obtained solid Si/C composite was thermally treated in argon atmosphere using induction annealing over the range of 1000–2400°C.

Characterization of the thermally treated Si/C via SEM/TEM/XRD revealed several material transformations at different temperature points. The sample treated at 1000°C displayed carbon coated silicon (Si@C) with approximate 10 nm carbon coatings, as well as interstitial carbon being present between the silicon nanoparticles. At 1400°C, due to the carbon diffusion into silicon, the previously observed Si@C transformed into silicon carbide nano particles (SiC-3C). Further increase of the annealing temperature (>1800°C) promoted fusion of 3C-SiC nano particles into larger structures and simultaneously propagated the sublimation of Si atoms from the SiC surface. Sublimation of Si atoms resulted in growth of graphite coatings on the surface of SiC at temperatures of 1800-2400°C. The sample obtained at 2400°C displayed 3 polytypes of SiC: SiC-3C, SiC-6H, SiC-15R and 2 polytypes of graphite: 3R and 2H and turbostratic graphite. The sample was composed mainly of graphite flakes with embedded SiC particles (10-100 nm) which displayed ~10 nm thick graphite coating.

Preliminary electrochemical testing in Li-ion coin cells, revealed a reversible capacity of 1200 mAh g^-1 for the Si@C sample, which also displayed an improved capacity retention after 50 cycles compared to the reference coin sell made from bare nano silicon particles.

References

[1] Y. Wen, Y. Zhu, A. Langrock, A. Manivannan, S.H. Ehrman, C. Wang, Graphene-bonded and -encapsulated Si nanoparticles for lithium ion battery anodes, Small. 9 (2013) 2810–2816. https://doi.org/10.1002/smll.201202512.

[2] M. lun Jiao, J. Qi, Z. qiang Shi, C. yang Wang, Three-dimensional Si/hard-carbon/graphene network as high-performance anode material for lithium ion batteries, J. Mater. Sci. 53 (2018) 2149–2160. https://doi.org/10.1007/s10853-017-1676-3.