Silicon and its oxides are attractive anode materials in energy storage devices, because of their high theoretical Li-storage capacities (Si: ~3580 mA h g⁻¹; SiO: ~2600 mA h g⁻¹; and SiO₂: ~1680 mA h g⁻¹), low lithiation potential (<0.5 V vs. Li/Li⁺), low cost and environmental benignity.^1,2 These silicon-based anodes can be used both in traditional lithium ion batteries and in more recent Li-S and Li-O₂ batteries as a replacement for the dendrite-forming lithium metal anodes.³ Currently, the most critical challenge is to improve their low initial coulombic efficiency and accommodate the structure degradation and instability of the solid-electrolyte interface caused by the large volume change. Here, we propose a binder-free structured Si/SiO_x anode that can tackle the above issues. We introduce a unique electrode fabrication technique based on directly encapsulating Si/SiO_x particles into cyclized polyacrylonitrile (PAN) frameworks as a monolithic, multi-core, cross-linking composite electrode. In this composite, the micro-nano Si/SiO_x particles reduced by high-energy ball-milling SiO act as active materials, and the cyclized PAN serves both as binders and conductive agents. Owing to the high electrochemical activity of Si/SiO_x and the good mechanical resiliency of cyclized PAN, this specific composite structure enhances the utilization efficiency of silicon oxides and accommodates its large volume expansion, as well as its good ionic and electronic conductivity. The cyclized-PAN coating Si/SiO_x composite exhibits excellent electrochemical properties, including a high initial reversible capacity (2734 mA h g⁻¹ with 75% coulombic efficiency), excellent cycle stability (988 mA h g⁻¹ after 100 cycles), and good rate capability (800 mA h g⁻¹ at 1 A g⁻¹rate). Because the composite is naturally abundant and shows such excellent electrochemical performance, it is a promising anode candidate material for lithium-ion batteries. The binder-free architectural design also provides an effective way to prepare other monolithic electrode materials for advanced lithium-ion batteries.

In Fig. 1A, the composite electrode exhibited a monolithic structure with coarse surface morphology, many Si/SiO_x particles were exposed onto the electrode surface, and several cracks were caused by the volume contraction of the PAN’s cyclization. These specific structural properties might guarantee the infiltration of the electrolyte into the bulk electrode, which was beneficial for charge transfer and ion transport. In Fig. 1B, the Raman spectra showed a sharp band at 502 cm⁻¹ which was attributed to the crystalline Si, indicating that nano-Si were dispersed into silicon oxides matrices, another two bands at 1356 and 1592 cm⁻¹ were attributed to the D-band (disorder-induced phonon mode) and G-band (graphite band), respectively, of delocalized sp² π bonds in nitrogen doped carbon. In Fig. 1C, the composite electrode showed a weak reduction peak at 0.52 V, which indicated that the capacity loss results from the formation of SEI, and that the reduction of SiO to Si could be negligible in the composite electrode. This was due to the existence of cyclized PAN layer coating on active materials that hindered the decomposition of the electrolyte, as well as the mechanochemical reduction of SiO to nano-Si oxides could reduce the formation of Li₂O and Li-silicates. Fig. 1D showed the charge–discharge profiles of the composite electrode at the current density of 0.1 A g⁻¹. It delivered a high initial discharge capacity of 2733.7 mA h g⁻¹ and a charge capacity of 2046.6 mA h g⁻¹, with 75% coulombic efficiency. The second discharge capacity could maintain at 2051.2 mA h g⁻¹. After 100 cycles (Fig. 1E), the composite electrode revealed a high capacity retention of 987.8 mA h g⁻¹, with an average coulombic efficiency of 98.5% for 100 cycles. Note that the cycle performance of the composite electrode was much better than that of the pristine SiO electrode.

In summary, a binder-free anode of cyclized-PAN coating Si/SiO_xcomposite was designed, and it exhibited excellent electrochemical properties in lithium ion batteries. This unique structural design would provide new avenues for the rational engineering of electrode materials for advanced lithium ion batteries and other electrochemical devices.

References

1 J.-I. Lee and S. Park, Nano Energy, 2013, 2, 146−152.

2 J. Wang, W. Bao, L. Ma, G. Tan, Y. Su, S. Chen, F. Wu, J. Lu and K. Amine, ChemSusChem, 2015, 8, 4073−4080.

3 N. Liu, Z. Lu, J. Zhao, M. T. McDowell, H.-W. Lee, W. Zhao and Y. Cui, Nat. Nanotechnol., 2014, 9, 187–192.

Fig. 1 (A) SEM image, (B) Raman spectrum, (C) Cyclic voltammograms, and (D) Charge–discharge profiles of the composite electrode, and (E) Cycle performance of the pristine SiO electrode and composite electrode at a current density of 0.1 A g⁻¹.