Insights into Li⁺ storage mechanisms, kinetics, and reversibility of defect-engineered and functionalized multi-walled carbon nanotubes for enhanced energy storage

Lithium-ion batteries (LIBs) are approaching their theoretical energy density limits due to the low capacity of electrode materials, and their charging rates are hindered by the intrinsically slow lithium cation (Li⁺) storage kinetics in graphite. To overcome these challenges, multi-walled carbon nanotubes (MWCNTs) have been explored as an alternative, offering Li⁺ storage within the interplanar space between graphene sheets, along with excellent electrical conductivity, and eco-friendliness. However, the defect-rich and functionalized configuration for reversible Li⁺ storage in MWCNTs is still the subject of debate. Here, we report the design and synthesis of defect-engineered MWCNT-COOH using an acid-treatment method. We conduct an extensive study of Li⁺ storage mechanisms, kinetics, and reversibility, by employing a suite of electrochemical and structural characterization techniques. The acid treatment successfully introduced extra Li⁺ storage active sites into MWCNTs, such as oxygen functional groups, structural defects, disordered carbon regions, voids/nanopores in the sidewalls, and uncapped hollow cores, as confirmed by Raman, XPS, and TEM analyses. These multiple active sites enable diverse pathways for Li⁺ storage, resulting in high overall capacities of up to 855.6 mA h g^?1 at 100^th cycle at 100 mA g^?1, surpassing the pristine MWCNTs with a capacity of 424.1 mA h g^?1 under the same conditions. Moreover, defect-engineered MWCNT-COOH exhibits good rate performance, delivering a capacity of 350 mA h g^?1 at 500 mA g^?1, as well as fast Li⁺ diffusion coefficients on the order of 10^?11 to 10^?10 cm² s^?1. The superior electrochemical performance of defect-engineered MWCNT-COOH allows for an increase in the energy density and a decrease in the charging time of LIBs, while maintaining a long lifetime and other performance metrics. Overall, this study provides crucial insights into Li⁺ storage mechanisms, kinetics, and reversibility of defect-engineered MWCNT materials and their synthesis for future battery designs.