A team from Pacific Northwest National Laboratory (PNNL), with colleagues from General Motors R&D, has developed a design principle for stable high-capacity Li-ion battery silicon (Si) anodes of controlled thickness swelling, and synthesized a porous Si/C-graphite electrode as an example.
Experimental data from the porous Si/C-graphite electrode showed excellent agreement with the theoretical design. The practical electrode (~3 mAh/cm2 loading) with ~650 mAh/g specific capacity had ~82% capacity retention over 450 cycles. The initial electrode swelling upon full lithiation is <20%. The calendered electrodes demonstrated ~56% end-of-life swelling and ~90% capacity retention after 200 cycles. A full-cell of Li(Ni⅓Mn⅓Co⅓)O2 and the pre-lithiated anode demonstrated >84% capacity retention over 300 cycles. A paper on the work is published in the RSC journal Energy & Environmental Science.
As has been noted many times, silicon has a high theoretical specific capacity of ~4200 mAh/g, making it a very attractive anode candidate for a high energy capacity Li-ion. However, the large volume expansion (>300%) during the lithiation process has posed commercialization challenges due to the resulting fast capacity fade of the electrode.
Many different approaches have been devised to addressing this problem, and significant progress has been made, the PNNL-GM team notes. However:
Despite of these efforts, electrode swelling of Si-based anodes, which is one of the most critical challenges for the practical application of batteries has been much less investigated. Limited swelling of yolk-shell structure Si-C composite electrode was demonstrated yet it is based on very thin electrodes. Recently, Si-nanolayer-embedded graphite prepared using the chemical vapor deposition of Si on graphite was reported to have ~517 mAh/g specific capacity and 96% capacity retention over 100 cycles with 50-cycle end-of-life swelling of ~38%. However, new approaches are still needed to further increase the specific capacity, improve the cyclability, and reduce the cost for large scale production of Si-based anodes.
In this study, we designed a porous Si/C-graphite electrode and used it as an example to elucidate how stable Si anodes with controlled swelling can be achieved.—Li et al.
The researchers synthesized the porous Si/C using electrochemical etching with controlled porous structure in which thin crystalline Si walls (~10 to 20 N·m) formed around large pores of up to ~50 nm in diameter. This was mixed with graphite at a controlled ratio targeting a specific capacity of ~650 mAh/g.
Upon full lithiation, the material exhibited a linear expansion of only ~13% along the pore direction and ~8% perpendicular to the pore direction comparing to the 60% linear expansion of Si nanoparticles.
By controlling the weight ratio of the porous Si/C in electrodes, the team could fine-tune the capacity and the cyclability for different applications.
Limited swelling of the porous Si/C-graphite electrode was demonstrated by in situ electrochemical dilatometer and corroborated by cross-section scanning electron microscopy (SEM) images of the electrodes before and after cycling.
The limited swelling and excellent electrochemical performance of the porous Si/C-graphite electrodes in half-cells and full-cells indicate that they have great potential as practical and stable high capacity anode required for next generation Li-ion batteries. Using porous Si by other scalable and economical methods, such as thermite reduction of SiO2 or chemical etching of metallurgical grade, micron-sized silicon powder, it is believed that practical Si-based anodes will enter the large scale market soon. The approaches for the rational design of practical electrodes developed in this work may also guide the development of other electrodes that may experience large volume changes during its operation processes.—Li et al.
Xiaolin Li, Pengfei Yan, Xingcheng Xiao, Jae Ha Woo, Chongmin Wang, Jun Liu and Ji-Guang Zhang (2017) “Design of Porous Si/C-Graphite Electrodes with Long Cycle Stability and Controlled Swelling” Energy Environ. Sci doi: 10.1039/C7EE00838D