Fragmentation of Carbon Nanotube Macro-Films By Ultrasound As Conductive Binder for Lithium-Ion Batteries

Fragmentation carbon nanotube macro-films (FCNT) with the structure of irregular laterally 2-D distributed CNT segments is found not only have high conductivity but also adhesive force. A conceptual idea to exploit FCNT alone as both binder and conductive additive for lithium-ion batteries was first-time proposed and demonstrated by constructing composite electrodes with a typical positive material, LiMn₂O₄ (LMO), in half cells. The new composite electrodes exhibit superior high-rate capabilities compared to the electrodes employing conventional binder, poly(vinylidene fluoride) (PVDF), and carbon black (CB). LMO with 30 wt% FCNT has specific capacity as high as 100 mAgh^-1 at 150 mAg^-1 and 78 mAhg^-1 at 1500 mAg^-1, which are about 87% and 70% of the 115 mAhg^-1 at an extremely low current density of 15 mAg^-1. In the contrast, the capacity of LMO with 15 wt% PVDF and 15 wt% CB drops over 55% of the initial capacity 87 mAhg^-1 when the current density rises 100 times from 15 mAg^-1 to 1500 mAg^-1. Electrochemical impedance spectra indicate 30 wt% FCNT contributes both the lowest series and charge transfer resistance out of the electrodes with 5 wt%, 15 wt% and 30 wt% FCNT, which determine its best high-rate performance. LMO with 30 wt% FCNT also maintains 100% retention for 100 cycle at 150 mAg^-1. An in-situ tribology method combining the wear track imaging and force measurement (shown in Figure 1) is introduced to evaluate the adhesion strength of electrodes comprising LMO with different mass fractions of FCNT and PVDF. The results show LMO with 30 wt% has adhesion strength of 7-10 MPa on the aluminum substrate which is at least comparable with the 6.3-8 MPa of 30 wt% PVDF. In addition, we further confirm the feasibility of this concept in two demo-systems, the half cells involving the negative materials Li₄Ti₅O₁₂ (LTO) and the full cells combining LMO-FCNT as cathodes with LTO-FCNT as anodes. FCNT could be a competent substitute for polymer binders and conductive additives to maintain mechanical integrity and electrical connectivity of active materials in the composite electrodes.