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Porous Silicon As Nanostructured Anode Material for Lithium Ion Batteries
Novel approaches to manufacturing processes and new kinds of anode materials for energy storage purposes are the key points to enable the effective and complete employment of technological platforms such as smart grid systems, electric vehicle and renewable energy sources. In this regard, it is urgent to overcome the limitations in energy density, process manufacturing and cost-effectiveness that hinder the existing battery technology from its full and complete development.
Recently, we have extensively reviewed the research advances in active nanostructured anode materials for the next generation of Li-ion batteries (LIBs). The anodes were classified into three groups: intercalation/de-intercalation, alloy/de-alloy and conversion materials [1]. In this framework, Silicon is a promising alloy/de-alloy material because it is earth-abundant and it holds a high theoretical gravimetric capacity (4200 mAh/gr) [2]. Moreover, silicon processability permits to obtain a nanostructured porous material (p-Si) with enhanced electrochemical hallmarks. In particular, p-Si can well fulfil the demanding requirements of novelty and effectiveness in anode materials.
The fabrication method of p-Si consists in a simple electrochemical etching that allows to tune the nanometric pore size by changing the etching parameters (current density, hydrofluoric acid concentration, silicon doping). Gravimetric porosity has been determined (from 30% up to 90%, Figure 1 - A-C) and the samples have been characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and Energy Dispersive X-ray spectroscopy (EDX) (Figure 1 – E-F). Furthermore, in order to characterize the mechanical behaviour that can have relevant effect on the cycling efficiency [3], different porosity samples have been investigated through nanoindentaion (Figure 1 – D). Finally, the p-Si (gravimetric porosity 80%) electrochemical performances have been preliminary evaluated obtaining, for the gravimetric capacity, the remarkable value of 900 mAh/gr. Interestingly, p-Si as anode material holds large margins of development especially in its rate capability.
To conclude, herein is presented an investigation on morphological, electrochemical and mechanical properties of nanostructured porous silicon, in the perspective of its employment as novel anode material. A cheap and scalable fabrication process has been set-up and the control over pore size has been achieved. The mechanical behaviour in term of Young module vs. porosity has been studied and the effective lithium storage capability has been demonstrated.
References
[1] Subrahmanyam Goriparti, Ermanno Miele, FrancescoDe Angelis, Enzo Di Fabrizio, Remo Proietti Zaccaria, Claudio Capiglia. 2013. “Review on recent progress of nanostructured anode materials for Li-ion batteries.” Journal of Power Sources in press.
[2] Wu, Hui, and Yi Cui. 2012. “Designing Nanostructured Si Anodes for High Energy Lithium Ion Batteries.” Nano Today(7): 414-429.
[3] Jung, Sung Chul, Jang Wook Choi, and Young-Kyu Han. 2012. “Anisotropic Volume Expansion of Crystalline Silicon During Electrochemical Lithium Insertion: An Atomic Level Rationale.” Nano Letters12 (10): 5342–5347.