Lithium-ion cycling performance of multi-walled carbon nanotube electrodes and current collectors coated with nanometer scale Al2O3 by atomic layer deposition

A carbon nanotube (CNT) electrode as a replacement anode current collector in a lithium-ion cell is a promising approach to increase cell gravimetric energy density and gain other operational benefits like flexibility or overdischarge tolerance. Concerns for substantial irreversible losses motivate the investigation of the electrochemical performance of a CNT electrode with a nanoscale coating of Al2O3 deposited by atomic layer deposition (ALD). Cycling of CNT electrodes in coin cells vs. Li metal between 0.005 and 1.5 V vs. Li/Li+ at 10 mA/g with a 1.2 M LiPF6 3:7 ethylene carbonate:ethyl methyl carbonate w/w electrolyte shows the Al2O3 coating leads to a 55% reduction in cumulative irreversible loss after 50 cycles. X-ray diffraction shows the Al2O3 coating promotes improved interlayer stability of the concentric graphene layers of the CNTs during cycling and Raman spectroscopy shows the Al2O3 limits the formation of defects in the CNT walls and/or amorphous carbon generation. In testing the Al2O3 coated CNT electrode as an anode current collector, cycling vs. Li metal of a mesocarbon microbead (MCMB) composite shows the Al2O3 leads to a 17% decrease in irreversible losses of the electrode after 5 cycles while electrode performance up to ∼ C/2 indicates the Al2O3 coating does not adversely affect current collector-to-composite charge transfer. Collectively, the results indicate that an ALD coating of Al2O3 is a promising way to decrease irreversible losses for CNT electrodes, representing an advancement towards use of CNT current collectors in lithium-ion cells.