High-Rate NMC Cathodes Achieved with Carbon Nanotube Additive
Through a collaborative effort, the Dillon Group at NREL and the Whittingham Group at SUNY -Binghamton have enhanced the conductivity of NMC cathodes to improve their rate capability in Li-ion batteries. These layered cathodes usually suffer from poor electrical conductivity and capacity fade at high charge/discharge rates. To mitigate these problems, the researchers have incorporated single-wall carbon nanotubes (SWNTs) into the NMC cathodes (LiNi0.4Mn0.4Co0.2O2). The resultant composite cathodes exhibit stable high-rate capacities, ~130 mAh/g at 5C and nearly 120 mAh/g at 10C for over 500 cycles, which are significantly higher than those achieved with conventional NMC cathodes.
- Figure 1. a) SEM image of NMC cathode with 5 wt.% carbon nanotube loading, revealing that the particles are interconnected with well-dispersed carbon nanotubes. b) TEM image of the composite electrode showing that the carbon nanotubes seamlessly stick on the NMC particle surface. c) Comparison of conventional NMC cathode and NMC cathode with carbon nanotubes (NMCCNT) for 100 cycles while varying from low to high rates and then returning to low rate.
The composite electrodes were 25-35 µm thick and contained 95 wt.% active material and only 5 wt.% SWNTs as a conductive additive. Scanning electron microscopy revealed that the carbon nanotubes infiltrate the entire structure, interconnecting the NMC nanoparticles (Figure 1a). The intimate contact between the SWNTs and active cathode particles was further confirmed with transmission electron microscopy, which showed a bundle of SWNTs conforming to the particle surface (Figure 1b). This configuration leads to the high electrical conductivity (verified with impedance spectroscopy) and suggests a possible chemical interaction between the SWNTs and the NMC particle surface.
Indeed, Raman spectroscopy indicated that charge transfer occurs between the SWNTs and the surfaces of NMC particles. Further evidence for this surface interaction was found in the form of larger lattice parameters observed with X-ray diffraction. This strong surface connectivity allows for the fast diffusion of electrons and lithium ions during cycling, resulting in a sustainable capacity at high rates. Figure 1c shows that the NMC-SWNT composite electrode has minimal capacity fade as compared to the conventional NMC electrode at both low and high rates. Furthermore, the composite electrode has a much higher coulombic efficiency, which indicates a higher surface stability.
This work was published in the January 1, 2011 issue of Advanced Energy Materials and was highlighted in Materials Views as one of the most significant scientific accomplishments in that issue.[1]
[1] C. Ban, Z. Li, Z. Wu, M. J. Kirkham, L. Chen, Y. S. Jung, E. A. Payzant, Y. Yan, M. S. Whittingham and A. C. Dillon. Advanced Energy Materials 1 (2011) 58-62.