Current LIBs [Li-ion battery] systems utilize graphite anodes, where energy is stored by intercalating lithium into the graphite layers. This arrangement, while commercially successful, can only deliver a maximum theoretical capacity of 370 mAh g⁻¹. Incorporating additional components offers the potential to dramatically improve this capacity, whereby silicon can provide up to 4,200 mAh g⁻¹ in theory. While Si-based composites offer immense promise as new generation anode materials, extreme changes in volume during lithiation and delithiation lead to structural degradation and debilitating performance loss over time that impedes their practical application.

Significant efforts have been devoted to tackling these problems by engineering Si-based electrodes at the nanoscale. … We have introduced the concept of using a flash heat treatment that dramatically improved the interfacial properties in the electrode design. However, the limitation in electrode loading and the high cost of high temperatures have led us to think of a new electrode design.

Herein we introduce a new electrode design concept that … involves wrapping SiNP [silicon nanoparticles] with S-doped graphene (SG), and then shielding this composite arrangement with cyclized polyacrylonitrile (PAN). … This provided a robust hierarchical nanoarchitecture that stabilized the solid electrolyte interphase (SEI) and resulted in superior reversible capacity of ~1,033 mAh g⁻¹ for 2,275 cycles at 2 A g⁻¹.

—Hassan et al.

Schematic of electrode process design. (a) Components mixing under ultrasonic irradiation, (b) an optical image of the as-fabricated electrode made of SiNP, SG and PAN, (c) the electrode after SHT, (d) Schematic of the atomic scale structure of the electrode. Source: Hassan et al. Click to enlarge.

The resulting material consists of micron-scale clusters in which the SiNP are well wrapped by SG and invariably dispersed within the nanosheets matrix.

The SG–Si delivers an initial discharge capacity of 2,865 mAh g⁻¹, based on all masses of SG, c-PAN and Si, with a high first-cycle Coulombic efficiency of 86.2%. (All capacities reported are based on the total mass of SG, c-PAN and Si.) The aerial charge capacity is about 3.35 mAh cm⁻²—close to the performance targets for next generation high-energy dense LIBs. Stable cyclability up to 100 cycles was obtained with an average capacity of 2,750 mAh g⁻¹.

A similar electrode structure, but prepared with non-doped graphene (i.e., G-Si), yielded an inferior rate capability and cycling stability; the high capacity of the G–Si persists only for 80 cycles, then fades gradually, reaching ~400 mAh g⁻¹ after 800 cycles.

The researchers attributed the capacity fading mainly to the degradation of the Si structure, where the expansion and shrinkage of SiNP during cycling leads to the separation from graphene scaffold, and subsequent loss of conductivity and instability in the SEI structure. They observed that the “significantly different electrochemical performances put a spotlight on the important role of sulfur in binding the SiNP to the surface of SG.”

In summary, the novel design of a Si-based electrode through the covalent binding of commercial SiNP and SG along with cyclized PAN offers exceptional potential in the practical utilization of Si anodes for LIB technologies. This covalent synergy enables superior cycling stability along with a high aerial capacity of the electrode, which is close to that of commercial technologies. Such a rational design and scalable fabrication paves the way for the real application of Si anodes in high-performance LIBs. The interaction between S and Si plays a critical role of improving the long-term cycle stability, in addition, the synergistic effect of the covalent bonds between Si–S, the facilitated charge transfer by 3-D graphene network and cyclized PAN and the improved electrode integrity all contributed to the superior cycle performance.

—Hassan et al.

Support for the work came from the Natural Sciences and Engineering Research Council of Canada (NSERC), the University of Waterloo, and the Waterloo Institute for Nanotechnology. Additional support came via the US Department of Energy (DOE) Office of Vehicle Technologies under contract no. DE-AC02-05CH11231, subcontract no. 7056410 under the Batteries for Advanced Transportation Technologies (BATT) Program.

Resources

• Fathy M. Hassan, Rasim Batmaz, Jingde Li, Xiaolei Wang, Xingcheng Xiao, Aiping Yu & Zhongwei Chen (2015) “Evidence of covalent synergy in silicon–sulfur–graphene yielding highly efficient and long-life lithium-ion batteries” Nature Communications 6, Article number: 8597 doi: 10.1038/ncomms9597