Micro-Sized Silicon-Carbon Composite Composed of Carbon-Coated Sub-10 Nm Si Primary Particles As High-Performance Anode Materials for Lithium-Ion Batteries

Recently, silicon has attracted considerable interest as a high-capacity anode material owing to its highest known capacity of 3580 mAh/g at room temperature, which is about 10 times larger than that of commercial graphite anodes (~370mAh/g). Despite promising advantages, practical application of silicon anodes in high performance Li-ion batteries is seriously hindered by its rapid capacity fading as a result of dramatic volume changes (>400 %) during lithium insertion and extraction processes. The huge volume changes of Si cause pulverization and loss of electrical contact between Si particles and conductive additives, as well as an unstable solid electrolyte interphase (SEI) layer, which in effect results in severe capacity fading. Constructing nanostructured silicon is a promising strategy and has been demonstrated to improve battery performance.^1,2 However, there are some challenges involved in introducing these nanomaterials into electrode fabrication for practical application. One is the lower tap density of the nanoparticles, in contrast to commercially used micro-sized electrode materials, resulting in a lower volumetric energy density; the other is the difficulty with handling the chemicals, such as inhalation and explosion risks, arising from these nanoparticles in fabrication of electrodes.Thus, Control of primary particle sizes of Si below critical diameter (around 10 nm) in the micro-sized composite is more important in achieving good cycling stability as micro-sized aggregated structures of Si can expedite the capacity fading.^3,4

Herein, we report a synthesis of micro-sized Si-C composites composed of carbon-coated sub-10 nm Si primary particles interconnected into secondary micro-sized particles, and its use as high-performance Li-ion anodes. This synthesis process involves facile thermal annealing of silsesquioxane with size-controlled formation of Si primary particles and subsequent carbon coating steps. The as-prepared Si-C composites anode material shows stabilized capacity of ~1600 mAh/g with excellent capacity retention (90% after 150 cycles). Moreover, the volumetric capacity can reach 1088 mAh/cm³ attributed to a high tap density (0.68 g/cm³) of micro-sized Si-C composites. Taking the facile synthesis and excellent performance of Si-C composites into account, it would be of significant interest for scalable fabrication for rechargeable lithium batteries. Furthermore, the silicon nanoparticles with controllable size are also easily used in other applications, such as light-emitting diodes, and high efficiency photovoltaic devices.

Acknowledgements

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract NO. 6951378 under the Batteries for Advanced Transportation Technologies (BATT) Program

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