Revealing the Mechanism of Electrochemical Lithiation of Carbon Nanotube Fibers

TL;DR

This study investigates the lithiation mechanisms of carbon nanotube fibers, revealing their high capacity and rate capabilities, while also identifying degradation processes like amorphization and the effects of defects.

Contribution

It provides a detailed analysis of the electrochemical and structural behavior of CNTFs during lithiation, highlighting the roles of surface processes, doping, and defects, with insights into improving stability.

Findings

CNTFs store lithium reversibly with high capacity and rate capability.

Electrochemical doping increases conductivity and causes Raman spectral shifts.

Amorphization and irreversible capacity are linked to defects and cycling conditions.

Abstract

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.