Advanced Multiphase Silicon-Based Anodes for High-Energy-Density Li-Ion and Li-Air Batteries

Given that lithium-ion and lithium-air batteries are the preferred choice for EVs and plug-in applications for the next 10–15 years, the focus is on improving their safety and performance. New, higher-capacity materials are required in order to address the need for greater energy density, longer cycle life and safer high-power operation. Silicon offers the highest gravimetric and volumetric capacity as an anode material (e.g. Li₂₂Si₅: nearly 4,200mAh/g, 9,800mAh/mL). The lithium-rich silicon compounds have high melting points.  Their higher working potentials (vs. Li) eliminate the possibility of metallic-lithium deposition due to overcharge. Silicon is the second-most  abundant  element  in  the  earth’s  crust,  and  it is  environmentally  benign. Unfortunately, Si-based electrodes typically suffer from large volume changes (up to 420%) during insertion and extraction of lithium. This is followed by cracking and pulverization of silicon, which in turn, leads to the loss of electrical contact, unstable SEI and eventual capacity fading.

We have developed two synthetic routes for the preparation of core-shell silicon multiphase composite particles. We studied the process of formation of the protective SEI layer on the anode nanoparticles and tested small laboratory-prototype cells with advanced anodes. The methods of attachment of silicon nanoparticles to carbon nanotubes (multi-wall carbon nanotubes  (MWCNT)) were developed with the purpose of creating silicon anodes supported by a strong, rigid and high-electrically-conducting matrix. The methods are based on the silanization process or pyrolysis. STEM analysis showed that silicon particles are dispersed uniformly on the surface of the MWCNT. ESEM images indicated the formation of silicon nanoparticles wrapped by carbon nanotubes and coated by nanometer thick amorphous carbon. TEM, EELS and XPS tests confirm the formation of a layer of carbon on top of the silicon nanoparticles and Si-C bonding (Fig.1). The feasibility of using electrophoretic deposition for the preparation of composite silicon anodes coated by a ceramic-in-polymer protective layer has been proven.

Li/LiPF₆ EC:DEC/Si-C-MWCNT cells with anodes composed of about 50% core-shell Si-C composite exhibited de-intercalation capacity of 1373mAh/gSi at the 50^th cycle and 1021mAh/cm² at the 410^thcycle. Li/LiTFSI-Pyr14/Si-C-MWCNT cells ran for more than 30 cycles with capacity of 1500mAh/gSi. Doped Si-alloy nanoparticles were formed by electrochemical means. The anodes are being tested.

Acknowledgments

Research is funded by EU FP7, contract no. 265971 and by Israel Academy of Science