Nanoporous Carbon Coating of Separator Boosts Rate Capability of Cathodes in Lithium-Ion Batteries

High rate capability of cathodes in lithium-ion batteries (LIBs) is essential for fast rechargeability of electric vehicles. Herein, we report that coating the cathode side of the separator with a layer of nanoporous carbon significantly improves the rate capability of LiNi_0.6Mn_0.2Co_0.2O₂ (NMC) and LiFePO₄ (LFP) cathode materials, where NMC and LFP deliver 121 mAhg^-1 and 125 mAhg^-1 at 2 Ag^-1 (∼12 C), respectively. To elucidate the underlying mechanism, we investigate the interfacial behavior and charge transfer kinetics using the distribution of relaxation times (DRT) from electrochemical impedance spectroscopy. Our DRT results show an increased relaxation time for mass transport and indicate a more populated charge storage in the diffuse layer of the electrical double layer (EDL). Simulations on ion transport behavior using a size-modified Poisson-Nernst-Plank equation reveal that the nanopore-confined EDLs produce an exclusion effect that increases Li-ion transport resistance in over-confined pores. These results support the experimentally observed increase in performance for coatings with expanded pore sizes and further indicate that the carbon layer enhances both (de)lithiation processes by promoting a more uniform and consistent gradient of Li-ions in the EDL.