Silicon Nanowire-Graphite Composites As High Energy Anode Materials for Lithium Ion Batteries
Authors: Keller Caroline , Karuppiah Saravanan , Kumar Praveen , Lapertot Gérard , Jouneau Pierre-Henri , Haon Cédric , Chenevier Pascale 
Published: May 2020

Silicon is one of the most promising candidates to replace commercial graphite anodes in the next generation of lithium-ion batteries. Although numerous efforts have been devoted to improve the electrochemical performance of the Si based anodes, scalable synthesis of such materials and retaining good cycling stability in high loading electrode remain unaddressed. In pursuit of this goal, we propose a simple and scalable approach to synthesize a novel architecture containing silicon nanowires (SiNWs) grown on carbon powder (graphite, graphene, carbon black). SiNWs are grown from gold nanoparticle catalysts deposited on carbon and from the thermal decomposition of an organosilane oil, diphenylsilane, at 430°C [1]. The SiNWs show thin, homogeneous diameters and a high aspect ratio (the left figure shows a typical SEM image). Composites including up to 70wt% of Si on carbon black and 40wt% Si on graphite are obtained at the gram scale within 2 hours.
SiNWs grown on graphite (Gt) anodes with 23 to 40 wt% of Si content demonstrated their relevance in next-generation high capacity anodes. This unique design can prevent the pulverization and crack by effectively accommodating the volume expansion during cycling. Consequently, Gt-SiNWs anodes exhibit a reversible capacity of 950 mAh g-1 after 180 cycles with 82 % capacity retention at C/5 rate. FIB-SEM cross sectional images of cycled anodes (detail shown on the right figure, 210 cycles) show that both Gt and SiNW are actively participating in (de)lithiation. As was shown in previous work on graphite-silicon nanoparticle composites, porosity increases in the graphite after cycling [2] correlates with the effective accommodation of volume change of SiNWs during cycling. More importantly, SiNWs are highly connected with graphite even after completing 210 cycles, while nanoporosity in the SiNWs aggregates is maintained.
Gt-SiNW anodes were cast with an electrode density of 1.5 g cm-3, compatible with industrial application. Even at 2C rate, Gt-SiNW anodes retain 87% of capacity after 500 cycles, showing a higher long-term stability than Si nanoparticle-carbon composites [3], even at various mass loading ranging from 0.8 to 2.7 mg cm-2, providing up to 2.5mAh/cm2. A full cell consisting of Gt-SiNW anode and NMC cathode delivers a high capacity of 17 mAh with 53% capacity retention after 250 cycles. Ongoing prelithiation studies are expected to increase the capacity retention by at least 30% by avoiding initial capacity loss. This work provides insights into the rational design of SiNWs anodes with wide variety of carbon substrates for high energy density batteries.
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Figure 1