Silicon is targeted as one of the more promising anode materials for next-generation lithium-ion batteries due to its relatively low working potential, abundance in nature, and theoretical gravimetric (specific) capacity of 3,579 mAh g⁻¹ for Li₁₅Si₄ at room temperature)—almost ten times that of commercialized graphite anodes.

However, as has been often reported, silicon experiences a significant change in volume (more than 300%) during the lithiation and delithiation processes, resulting in pulverization and loss of electrical contact, as well as formation and propagation of an unstable solid electrolyte interphase (SEI) on its surface—both of which result in rapid capacity fading of the battery.

Compared with Si bulk films and/or micrometer-sized Si particles, the nanostructuring of Si represents a very popular tactic to deal with the volume change-induced instability issues of Si, as well as has led to a quantum leap in the lithium storage properties in terms of gravimetric capacity, rate capability, and/or cycling stability.

… Unfortunately, such transformative nanoscale materials design principle has generally been plagued by the relatively low tap (packing) density of Si, which first lowers volumetric capacity reflecting volumetric energy density, and second increases the use of other auxiliary components (e.g., binder, conductive carbon, foils) of the battery capable of reducing the gravimetric capacity and hence gravimetric energy density, fatally limiting the implementation of Si in a viable lithium ion battery.

… a major challenge is to build an energetic electrode architecture that can address the aforementioned volume change-induced problems and simultaneously realize the trade-off between the increase of the Si tap density and the incorporation of the required void space. We believe that the similar situation holds inevitably for other emerging electrode materials subject to large volume changes. … in this contribution we propose and develop a novel electrode configuration where the well-ordered perforated Si nanowires are conformally coated with the graphitic carbon sheets (G) and interconnected through an underlying hinging layer, thus forming a mechanically robust, free-standing textured silicon@graphitic carbon nanowire (namely, t-Si@G NW) array.

—Wang et al.

Electrochemical cycling performance and Coulombic efficiency of the t-Si@G NW array electrodes under charge/discharge cycles from 2 to 0.02 V. Cycling performances of control electrodes (Si NW and t-Si@G NW) are compared in the figure. Credit: ACS, Wang et al.Click to enlarge.

The research team reported that the resulting electrode exhibits “unprecedented lithium storage performances”, especially in terms of volumetric capacity. They also reported that, to the best of their knowledge, this work marks the first time that a binder-free silicon-based lithium-ion battery anode with both high gravimetric capacity and high volumetric capacity was designed and exploited from the viewpoint of both materials unit and whole electrode (materials ensemble).

They attributed the performance to a number of advantages of the material:

• The ordered one-dimensional (1D) nanowire array—demonstrated by others with silicon as well as carbon and other materials—allows for a substantially high tap density of Si. The ordered 1D nanowire arrays also reduce tortuosity in electronic and ionic transport and thus improve the rate capability, as other studies have also shown.

• The void space, which is incorporated uniformly within the whole array to accommodate the volume change of Si, is around two times the Si volume, so as to avoid the failure of the electrode structure and cell configuration due to the inherent volume expansion of Si, as well as to avert the excessive incorporation of the void space at the cost of compromising the tap density of Si.

• Each perforated Si nanowire is composed of interconnected pore walls with an average thickness of ∼8 nm, which facilitates strain relaxation and enhances tolerance to the volume change of Si, thereby ensuring the structural integrity of materials units upon cycling. This also is in agreement with recent studies that have suggested a critical size (∼10 nm) with which the silicon is structurally stable in terms of exhibiting the optimal electrochemical performance.

• The graphitic carbon coating mimics the modality of Si completely, thus improving the Si interface with the electrolyte. In addition, the graphitic carbon inside and outside the Si nanowires is interconnected by an underlying hinging layer, thus affording efficient pathways to promote lateral electron transport from/to each nanowire. More importantly, they add, such a thin carbon coating enables the construction and direct utilization of free-standing t-Si@G nanowire arrays without the addition of any other additives or substances.

On the basis of the nanoscale system engineering formula, we have demonstrated its direct use as a high-performance anode with high volumetric capacity on the basis of the whole electrode, competing rate capability, and excellent cycling stability … These performance parameters are achieved without the expense of disturbing other components of the battery, attributing to the incorporation of an indispensable void space at the whole electrode scale. Coupled with a simple and scalable production protocol, the electrode architecture developed represents a significant advance in developing Si-based electrodes practically applicable to next-generation lithium ion batteries. More importantly, the nanoscale system engineering formula high- lighted could also be extended to other emerging high-capacity anode and cathode materials that undergo large volume-expansion.

—Wang et al.

Resources

• Bin Wang, Xianglong Li, Tengfei Qiu, Bin Luo, Jing Ning, Jing Li, Xianfeng Zhan, Minghui Liang, and Linjie Zhi (2013) “High Volumetric Capacity Silicon-Based Lithium Battery Anodes by Nanoscale System Engineering” Nano Lett. doi: 10.1021/nl403231v