  Fabrics of continuous fibers of carbon nanotubes (CNTFs) are attractive
materials for multifunctional energy storage devices, either as current
collector, or as active material. Despite a similar chemical composition,
lithiation/delithiation in CNTFs is substantially different from traditional
graphite electrodes. In CNTFs this process is dominated by surface processes,
insertion in the bundles interstices, electrochemical doping and
often-overlooked partial degradation of the sp2 lattice upon cycling. Through
extensive electrochemical analysis, together with in situ Raman spectroscopy
measurements, we analyzed the complex lithiation behavior of highly crystalline
fibers of CNTs. CNTF can store lithium reversibly with high specific capacity
and rate capability, thanks to a large capacitive contribution. Upon
lithiation, they undergo electrochemical doping, with longitudinal conductivity
increasing by as much as 100 %, concomitant with large downshifts in Raman
spectra. However, CNTF are also affected by high first-cycle irreversible
capacity, voltage hysteresis and amorphization upon cycling. Electrochemical
analysis confirms that SEI formation is responsible for the first-cycle
irreversible capacity. Voltage hysteresis is attributed primarily to the
trapping of lithium ions in the interstices between stacked nanotubes. Another
dominant feature is pre-existing defects, which promote capacitive storage but
lead to progressive amorphization of the CNTFs. Indeed, it is evidenced that
undesired amorphization is hindered in ultra-pure CNTF without pre-existing
defects.
