Spontaneous local redox reaction to passivate CNTs as lightweight current collector for high energy density lithium ion batteries

Extensive usage of highly conductive carbon materials with large specific surface area (e.g., carbon nanotubes, CNTs) in lithium ion batteries (LIBs), especially as current collector of anodes, suffers from low initial coulombic efficiency (ICE), large interfacial resistance, and severe embrittlement, as the large specific surface area often results in severe interfacial decomposition of the electrolyte and the formation of thick and fluffy solid electrolyte interphase (SEI) during cycling of LIBs. Herein, we demonstrate that when the CNT-based current collector and Na foil (which are being stacked intimately upon each other) are being placed in Na+-based organic electrolyte, local redox reaction between the Na foil and the electrolyte would occur spontaneously, generating a thin and homogeneous NaF-based passivating layer on the CNTs. More importantly, we found that owing to the weak solvation behaviors of Na+ in the organic electrolyte, the resulting passivation layer, which is rich in NaF, is thin and dense; when used as the anode current collector in LIBs, the pre-existing passivating layer can function effectively in isolating the anode from the solvated Li+, thus suppressing the formation of bulky SEI and the destructive intercalation of solvated Li+. The relevant half-cell (graphite as anode) exhibits a high ICE of 92.1%; the relevant pouch cell with thus passivated CNT film as current collectors for both electrodes (LiCoO2 as cathode, graphite as anode) displays a high energy density of 255 Wh kg−1, spelling an increase of 50% compared with that using the conventional metal current collectors.