Li-ion rechargeable batteries (LIBs) are the most promising candidates for use in electric and hybrid electric vehicles (EVs and HEVs) due to their high operating voltage and superior energy density compared to other conventional batteries such as the Ni-metal hydride battery. To enable cost-effective and long-lasting EVs, DoE estimates that the performance of present battery systems must be improved by at least four times without increasing the cost. LiFePO₄ (LFP) emerged as a competitive cathode material for next-generation LIBs due to its remarkable stability and non-toxicity but they suffer from low electrical conductivity. While the addition of carbon improves the in-plane electrical conductivity, it fails to provide a conducting interface between the LFP/C/binder film and the current collector. This interfacial resistance at the current collector and active material interface (CCAMI) is critical for achieving high power density and rate capability but is often neglected. We addressed this issue by engineering the CCAMI with carbon nanotubes (CNTs). Previously, we demonstrated two roll-to-roll binder-free processes for coating Al foils with CNTS: (i) a CVD-based process for directly growing vertically aligned CNTs (VACNTs) on bare kitchen-grade Al foils [1], and (ii) a spray-coating process for coating industrial-grade Al foils with randomly oriented CNTs [2]. The above mentioned processes eliminate the need for a binder and thereby reduce both the dead weight of the inactive material and the CCAMI resistance. Specifically, we found that the VACNTs- or randomly oriented CNTs-coated Al foils obtained via our roll-to-roll processes enhance the areal (/gravimetric) capacity of LFP by >65% (/>50%) at low C-rates (<2 C), and by >85% (>70%) at high C-rates (>2 C). Moreover, the improved CCAMI resulted in gravimetric energy densities up to 360 Wh/kg and power densities up to 200 W/kg with much higher power capability (increased charge capacity at high discharge rates). Thus, this study describes an attractive approach for improved CCAMI, which is scalable and compatible with existing industrial protocols for coating LFP and takes us many steps closer to the commercial deployment of LIBs in HEVs and EVs.

[1] M. R. Arcila-Velez et al., Nano Energy. 8 (2014) 9–16. doi:10.1016/j.nanoen.2014.05.004.

[2] M. Karakaya et al., Appl. Phys. Lett. 105 (2014). doi:10.1063/1.4905153.