Correlations of Precursor and Carbon Coating with Electrochemical Property of 2D Graphitic Carbon Nitride (g-C3N4) Nanosheets as High-Reversibility Cathode of Nonaqueous Al-Ion Batteries

Al-ion batteries (AIBs) represent promising multivalent-ion battery alternatives to Li-ion batteries (LIBs). Exploration of cathode materials is the key to improving the electrochemical performance of AIBs. Here we demonstrate a promising AIB cathode candidate based on two-dimensional graphitic carbon nitride (g-C3N4) nanosheets with reversible de/embedding of [AlCl4]- anions in its special layered porous structure during the cyclic charge/discharge process. To address the sluggish ion/charge transportation kinetics that is inherently present in AIBs due to the intrinsically low conductivity of g-C3N4, we optimize different synthetic precursors of g-C3N4 including dicyandiamide, urea, and melamine and, further, develop a facile method for uniformly coating a N-doped carbon (N-C) coating layer on the optimal g-C3N4 derived from dicyandiamide. The resultant AIBs consisting of the g-C3N4@N-C cathode and Al metal anode in the AlCl3/[EMIm]Cl electrolyte exhibit an initial capacity of similar to 54.9 mAh center dot g-1 at 0.2 C with a high Coulomb efficiency similar to 99.9%, and they possess long-term cycling over 200 cycles. The underlying charge/discharge process of AIBs, revealed by the ex situ X-ray diffraction and scanning electron microscopy analyses, involves the extraction/insertion of Al species (mainly Al3+, AlCl4-, and Al2Cl7-) into the g-C3N4 crystal lattices with the formation of intermediate Al8C3N4 and (ClCN)3.