High-capacity electrode materials are inherently accompanied by large volume changes that pose a significant, multifaceted challenge to their functions in rechargeable batteries. At the apex of this challenge is silicon, which has a theoretical capacity as high as 3579 mAh/g at room temperature, ∼10 times that of graphite anodes used in lithium ion batteries. However, the large volume change (∼270%) during lithium ion insertion/extraction induces enormous mechanical strain that causes pulverization of Si, loss of electrical contact, and uncontrolled growth of solid electrolyte interphase (SEI), resulting in rapid decay of capacity. Over the past decade a number of elegant strategies have emerged to address the various aspects of this multifaceted challenge. … However, it remains an unmet goal to harness Si’s potential. Particularly, it is known that at high mass loadings the structural integrity and electrical interconnection become exceedingly difficult to maintain during electrochemical cycling.

… Here, we show for the first time that chemical tailoring of the nanostructure interface with atomic oxygen can substantially improve the electrochemical performance of silicon/carbon nanotube (CNT) composite electrodes. Due to the inherently weak adhesion between Si and the carbon lattice, a persistent challenge for nanocomposites in general, Si detaches from the conductors and agglomerates during repeated electrochemical cycling. By chemically tailoring CNTs with atomic oxygen, we found that the poor interface between Si and carbon is significantly improved. Owing to this robust interface, Si stays firmly immobilized on CNTs, effectively blocking the delamination and agglomeration. Both structural integrity and electrical connectivity are well maintained throughout the entire electrode, thereby offering superior electrochemical performance.

—Sun et al.

Schematic illustration of stabilizing Si anode by atomic oxygen-tailored interface. (a) Si grows on CNTs as particle aggregates, which detach from the conductive support during repeated electrochemical cycling. (b) Oxygen-containing moieties introduced by UV−ozone enable uniform nucleation of Si on CNTs. With robust interfacial bonding, Si adheres firmly on CNTs during electrochemical cycling. Credit: ACS, Sun et al. Click to enlarge.

They used a simple dry chemistry, applying UV−ozone (UVO) to CNT yarns with controlled porosity. They found that UVO produces atomic oxygen capable of functionalizing CNTs only on the outer walls. This surface-limited feature is distinctly different from oxygen plasma, the highly energetic ionic species of which are penetrative and destructive, and oxidative wet chemistry. By tuning the exposure time of UVO, they can achieve the desired degree of surface functionalization of CNTs.

The tailored Si/C interface significantly improved electrochemical cycling performance: a capacity of 922 mAh/g based on the mass of the entire electrode, or 4 times higher than the capacity of graphite electrode (229 mAh/g). The volumetric capacity was 737 mAh/cm³, which is also higher than that of graphite anodes (600 mAh/cm³). This value, the researchers noted, could be further improved by reducing the CNT diameter and controlling the packing density of the CNT yarns.

With the interfacial bonding, the composite delivered a superior cycling stability up to 500 cycles (1.62 mAh/cm² at the 500th cycle). The capacity retentions were 91.6% at the 150^th cycle, 78.8% at the 300^th cycle, and 67.1% at the 500^th cycle—among the highest reported at a high mass loading of Si.

Improved electrochemical performance. (a,b) Voltage profiles at a lithiation/delithiation rate of C/5 (1C = 4.2 A per gram of Si). (c) Galvanostatic cycling performance of CNT@Si control and f-CNT@Si (left vertical axis) and delithiation areal capacity of f-CNT@Si (right vertical axis) at a rate of C/5. (d,e) Galvanostatic cycling performance of f-CNT@Si and control (Coulombic efficiency (d) and delithiation areal capacity (e)) at a rate of C/5 for the first through third cycles, C/2 for the fourth through sixth cycles, and 1C for all subsequent cycles (FEC electrolyte was used). All specific capacities were calculated based on the mass of the entire electrode. Credit: ACS, Son et al. Click to enlarge.

Resources

• Chuan-Fu Sun, Hongli Zhu, Morihiro Okada, Karen Gaskell, Yoku Inoue, Liangbing Hu, and YuHuang Wang (2014) “Interfacial Oxygen Stabilizes Composite Silicon Anodes,” Nano Letters doi: 10.1021/nl504242k