PNNL team develops silicon composite with conductive rigid skeleton as stable high-capacity Li-ion anode material
20 July 2012
In the ongoing search for a stable silicon material for use as a high-capacity anode for next-generation Li-ion batteries, a team from Paciﬁc Northwest National Laboratory (PNNL) has developed a cost-effective and scalable method to prepare a core–shell structured silicon composite with a conductive, rigid boron carbide (B4C) skeleton.
The Si/B4C composite is coated with a few graphitic layers to further improve the conductivity and stability of the composite. The Si/B4C/graphite (SBG) composite anode shows excellent cyclability with a specific capacity of 822 mAh·g–1 (based on the weight of the entire electrode, including binder and conductive carbon) and 94% capacity retention over 100 cycles at 0.3 C rate. A paper on their work is published in the ACS journal Nano Letters.
In the past decade, great efforts have been made to reduce the Si particle size or produce three-dimensional (3D) porous electrode structures to avoid the pulverization and improve the stability of the Si-based anodes. Various nanostructures are synthesized, including Si nanowires8 and conductive-core/Si- shell core−shell structured nanowires fabricated by chemical vapor deposition (CVD), 3D porous Si anodes prepared by chemical deposition of Si on templates, tobacco mosaic virus (TMV) templated 3D Si anodes, carbon or polymer scaffold-supported Si anodes, and Si-carbon nano- composite granules formed through hierarchical bottom-up assembly.
Although some of the properties of these S-based anodes far exceed those of the conventional anode materials and demonstrated very high capacity and good stability (especially those of the nanostructured Si nanowires or hollow structures), overall performance (especially the electrode loading or the capacity per unit area) of these Si-based anode materials still cannot satisfy the needs for practical applications.
Another significant challenge is to develop materials and methods that are cost-effective and applicable for large-scale manufacturing.—Chen et al.
|Long-term cycling stability of SBG433 at a current density of 0.63 A·g−1. Credit: ACS, Chen et al. Click to enlarge.|
The PNNL team developed a silicon-boron carbide (B4C)-graphite (SBG) core−shell−shell structured composite using simple ball-milling (BM) of commercially available materials. The highly conductive boron carbide is used as nano/micro-millers to break down micron-sized Si and also as a conductive rigid skeleton in the final composite to support the in situ formed sub-10 nm Si particles to alleviate the volume expansion during charge/discharge. The graphitic coating on the Si/B4C composite improves the electrical conductivity and favors formation of the solid electrolyte interphase (SEI) film.
The specific capacity of the SBG composite is 822.5 mAh·g−1 at 0.63 A·g−1 and 601.2 mAh·g−1 at 2.50 A·g−1 based on the weight of the entire electrode including the weight of binder and conductive carbon.
The specific capacity based on Si is 2,938 mAh·g−1 at 0.63 A·g−1 and 2147 mAh·g−1 at 2.50 A·g−1. The SBG composite with an optimized graphite shell exhibits an initial Coulombic efficiency of 82.3%.
The approach of using conductive hard material as nanomillers to in situ form sub-10 nm Si particles and subsequently form a conductive-rigid-skeleton-supported Si-based composite can be a general method for fabricating high performance electrodes for Li-ion batteries. Both the source materials and the preparation approaches used in this work are cost-effective and easy to scale up. Therefore, it has a good potential to be used for large scale applications. Furthermore, the approach reported in this work can also be used to stabilize other functional nanocomposite materials which may experience large volume expansion during physical, chemical, or electrochemical operations.—Chen et al.
Xilin Chen, Xiaolin Li, Fei Ding, Wu Xu, Jie Xiao, Yuliang Cao, Praveen Meduri, Jun Liu, Gordon L. Graff, and Ji-Guang Zhang (2012) Conductive Rigid Skeleton Supported Silicon as High-Performance Li-Ion Battery Anodes. Nano Letters doi: 10.1021/nl301657y
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