Three groups under DOE BATT program independently synthesize silicon-based high-capacity anode materials for Li-ion automotive batteries
Three groups funded under the US Department of Energy (DOE)’s Batteries for Advanced Transportation Technologies (BATT) program (earlier post) have independently synthesized silicon-composite nanostructures as Li-ion anode materials with excellent capacity retention with greater than 800 mAh/g reversible capacity. Silicon is of great interest as a high-capacity anode material, but has been hampered for commercial automotive applications because of its short lifecycle and capacity degradation. (Earlier post.)
Prashant N. Kumta, University of Pittsburgh; Ji-Guang Zhang and Jun Liu at Pacific Northwest National Laboratory; and Yi Cui at Stanford University are pursuing different approaches to developing alternative silicon-based anode materials to replace graphite to provide higher gravimetric and volumetric energy density in Li-ion batteries. Their progress is outlined in the First Quarter FY 2012 Report for the BATT program.
Kumta. Kumta is searching for inexpensive silicon, carbon, and other inactive matrix based composite powders that provide 1) an electrochemical potential a few hundred mV above the potential of Li, and 2) a capacity of 1200 mAh/g or greater (>2600 mAh/ml). The research is focused on exploring novel economical methods to generate nanoscale heterostructures of various Si nanostructures and different forms of C derived from graphitic carbon, nanotubes (CNT) and new binders.
His team has synthesized nano-scale electrodes comprising Si-graphitic carbon-polymer derived C, and carbon nanotube (CNT) related systems; the nano-composite Li-Si-C hetero-structures exhibit stable capacities of 700-3000 mAh/g.
By the end of the third quarter 2012, Kumta expects to have generated nano-composite core-shell, random, and aligned structures of varying nanoscale Si morphologies, boron (B), and C nanotubes exhibiting 1500 mAh/g and higher capacities. The group is working to generate novel binders, study the synthesis conditions, nano-scale microstructure affecting the energy density, rate capability, first cycle irreversible loss and coulombic efficiency, characterize the SEI layer, and outline steps to yield stable capacity, reduce irreversible loss and increase coulombic efficiency.
Zhang and Liu. The approach of Zhang and Liu is to manipulate the nano-structure and conductivity of silicon (Si)-based anodes to improve their mechanical and electrical stability.
One approach has been to investigate porous silicon with micrometer particle sizes and different nanopore sizes.The porous Si powders have been coated with a thin layer (~6% in weight) of carbon by chemical vapor deposition (CVD) to increase their electrical conductivities. The porous structure of Si helps to accommodate the large volume variations that occur during Li-insertion/extraction processes.
So far, they have obtained an initial capacity of ~1200 mAh/g (based on the full electrode) and capacity retention of ~800 mAh/g over 30 cycles at a 0.1C rate with that material.
In another effort, they developed a SiC/SiO/C core-shell composite, which showed an initial capacity of ~1000 mAh/g (based on the full electrode) and capacity retention of ~600 mAh/g over 100 cycles.
By the end of Q3 2012, they expect to have prepared micro-sized Si particles with large nano-pores will be prepared, with a targeted initial capacity of >1000 mAh/g (based on electrode) and capacity retention of ~700 mAh/g over 100 cycles will be obtained. They will investigate a SiC/Si/C (or SiC/SiO/C) core-shell composite further to improve its performance with a targeted initial capacity of >1200 mAh/g (based on the full electrode) and capacity retention of ~700 mAh/g over 100 cycles.
Cui. Cui and his team at Stanford are exploring new types of Si nanostructures, specifically, a variety of hollow and porous nanostructures. They have fabricated a variety of spherical, one-dimensional, tubular, and porous Si nanostructures into Si anode architectures.
Enclosing Si nanoparticles in a thin tubular carbon shell within which there is space for unimpeded volume expansion during lithiation allowed for the outer carbon layer to remain intact even after full lithiation and expansion of the Si nanoparticles. This composite material showed excellent cycling behavior in Li half-cells, retaining >85% of the initial discharge capacity after 200 cycles with CE >99% after the initial 50 cycles.
By the end of Q3, Cui expects to have optimized anode cycle life, Coulombic efficiency, first cycle irreversible capacity loss, specific capacity, and mass loading by varying synthesis conditions.