Synthesis and Investigation of Surface-Modified Silicon Nanoparticles As Advanced Anodes for Li-Ion Batteries
Authors: Jesch Rebekka , Omar Ahmad , Oswald Steffen , Guo Jing , Mikhailova Daria , Giebeler Lars 
Published: February 2020

Since their invention in the 1990s, Li-ion batteries (LIBs) have been the emergent battery technology, and have been instrumental towards widespread technological revolution in the present-day society. Commercial success aside, the energy density and the cycle life of state-of-the-art LIBs still cannot keep up with the needs of the modern world, e.g. in elctromobility [1]. Thus, there is a strong thrust towards development of advanced LIBs and Si based systems are one of the more promising technology[2], as they have significantly high theoretical specific capacity (3579 mAh/g) more than 10 times that of commercial graphite.[3] Furthermore, Si being low cost and environmentally friendly, makes them quite attractive for applications. Unfortunately, with regards to silicon chemistry, significant challenges inhibiting long term performance are still unresolved which act as hurdles toward broad commercialization. Si undergoes a large volume change (~300 %) during lithiation/delithiation, causing significant degradation of electrode with cycling in the form of cracking, contact loss and delamination. This inevitably leads to loss of active material and low cycling efficiency [3]. Nanostructuring of Si anodes has proven to be a successful strategy towards mitigating the stress involved, thereby minimizing the electrode degradation [4,5]. However, in nanoscale materials with high surface to volume ratio, the solid-electrolyte interphase (SEI) becomes a critical factor. Moreover, the SEI formed on silicon surface, in conjunction with common electrolytes, is relatively unstable and undergoes increasing resistance and acts as the bottleneck towards stable performance. In this work, we present our results on synthesis and functionalization of silicon nanoparticles (SiNPs). The aim is to influence the formation of the SEI by selectively modifying the surface of SiNPs Hydrogen silsesquioxane, obtained from hydrolysis of trichlorosilane, was subsequently annealed and etched to obtain sub-10nm sized SiNPs. The silicon surface was then modified with linear hydrocarbons containing different functional groups. Different functional groups (carboxyl group and amine group) were chosen to systematically study the effects on SEI formation and binder-interaction. Detailed characterization at different steps of the synthesis was done using FTIR, XRD and SEM. XPS and TEM was used to study the structure and morphology of the surface modified SiNPs. The SiNPs were prepared as electrodes and their electrochemical performance was evaluated versus lithium (half-cells) using LP30 as an electrolyte (Figure 1). Thus, an insight into the interaction of functional surface towards SEI formation, shall be instrumental towards development of a highly efficient and stable SEI for nanostructured Si anodes with high energy density and long term cycling stability. We thus present a relatively new approach, with large possibilities for further work, which is interesting and promising to leverage the power of silicon towards LIBs.
Figure 1: a) Specific capacities (open symbol indicates discharge capacity and filled symbol indicates charge capacity) and b) Coulombic efficiencies for different functionalized SiNP anodes along with non-functionalized SiNP anodes in half cells with LP30 (1M LiPF6 in Ethylene carbonate:Dimethyl carbonate 1:1 v/v) as an electrolyte.

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Figure 1