As the world moves away from distributed fossil fuel use in order to mitigate the climatic effects of carbon pollution, the need for high energy density storage devices continues to grow. Secondary lithium ion batteries (LiB) are one such attractive energy storage device. Current LiB technology relies on graphitic carbon as the anode material, with a theoretical capacity of 372 mAh/g. In order to increase the energy density of LiBs, anode materials with a greater capacity for lithium storage are under intense investigation. Materials which form alloys with lithium such as antimony, germanium, silicon, and tin, all have theoretical capacities which far surpass graphite. However silicon, as the most naturally abundant element and possessing a theoretical capacity of 3579 mAh/g in the Li₁₅Si₄ alloy, is the most promising for global adoption in next generation LiBs.

There are issues which require resolution before silicon can be implemented. Large volumetric changes associated with the lithiation-delithiathion process (~300%) result in material pulverization and loss of electrical contact [1]. Also unstable solid-electrolyte-interphase (SEI) formation during cycling results in the consumption of lithium during operation and capacity fade [2]. Previous studies have conclusively shown that the former issue may be mitigated by utilizing nano-scale silicon materials, with particles under 150 nm in diameter remaining intact during the swelling and contraction associated with cycling [3]. It has also been demonstrated that by encapsulating the silicon materials in carbon shells shows promise in stabilizing the SEI. Highly rational silicon-carbon architectures have been developed to accomplish this, such as the carbon clamped hollow silicon nanoshphere, “yolk in shell”, and pomegranate geometries [4] which achieve high capacity and robust performance over hundreds of cycles. Yet these complicated structures pose difficulties in production at the industrial level, requiring multiple synthesis steps and pre-lithiation in some cases. For mass adoption, a scalable process for generating carbon coated silicon nanomaterials is required.

One such process is laser mediated pyrolysis, which has the capability to produce kg/h of high purity nanoparticles within a narrow size distribution. The technique, which has been used to produce various ceramic, oxide, and metallic particles [5], has already been utilized on the industrial scale for silicon nanoparticle production. In this work we present a novel two stage pyrolysis reactor which synthesizes silicon nanoparticles in the first stage and adds a nanometric carbon shell in the second stage. The technique avoids any manipulation of the pure silicon powders prior to carbon coating, mitigating oxidation and particle degradation. Furthermore we discuss techniques to control particle size and coating thickness in order to optimize material performance. The capacity of current crystalline silicon core-carbon shell materials reaches ~2500 mAh/g at C/10 and retains over 70% capacity at a 2C rate over 500 cycles. Methods to produce amorphous silicon particles and their improved capacity retention will also be discussed. Physico-chemical characterization of these materials and battery performance will also be presented. Furthermore we elucidate steps to mitigate first cycle irreversible behavior associated with high surface area materials such as electrode ink formulation, nanoparticle hybridization, and chemical integration with secondary carbon structures.

[1] M.N. Obrovac, V.L. Chevrier. Alloy Negative Electrodes for Li-Ion Batteries. Chem. Rev., 2014, 114.

[2] N. Liu et. al. A Yolk Shell Design for Stabilized and Scalable Li-ion Battery Alloy Anodes. Nano Lett., 2012, 12.      

[3] X. H. Liu et. al. Size Dependant Fracture of Silicon Nanoparticles During Lithiation. ACS Nano, 2012, 6. 

[4]  H. Wu, Y. Cui. Designing Nanostructured Si Anodes for High Energy Lithium Ion Batteries. Nanotoday, 2012, 7.

[5] N. Herlin-Boime et. al. Synthesis of Covalent Nanoparticles by CO₂ Laser. Encyclopedia of Nanoscience and Nanotechnology, American Scientific Publishers, 2004, 301-326.