U Maryland team devises new method to stabilize high-capacity Si anodes for Li-ion batteries: interfacial oxygen
|Interfacial oxygen between the silicon and carbon improves electrode performance. Credit: ACS, Son et al. Click to enlarge.|
Researchers at the University of Maryland have improved the cycle life of silicon/carbon matrix-composite electrodes by 300%, even at mass loadings, solely by the chemical tailoring of the interface between the silicon and the carbon with atomic oxygen.
The interface-tailored electrodes simultaneously attained high areal capacity (3.86 mAh/cm2); high specific capacity (922 mAh/g based on the mass of the entire electrode); and excellent cyclability (80% retention of capacity after 160 cycles)—among the highest reported. Even at a high rate of 1C, the areal capacity approaches 1.61 mAh/cm2 at the 500th cycle. In a paper in the ACS journal Nano Letters, the team sugests that interfacial bonding—“a new dimension that has yet to be explored,”—can play an unexpectedly important role in addressing the multifaceted challenge of Si anodes.
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.
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 150th cycle, 78.8% at the 300th cycle, and 67.1% at the 500th cycle—among the highest reported at a high mass loading of Si.
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