Energy Storage Mechanisms in High-Capacity Graphitic C3N4 Cathodes for Al-Ion Batteries

Al-ion batteries are a promising alternative to lithium-ion batteries because of the unique advantages of the Al anode, such as low cost and high specific capacities. Cathodes developed for these batteries, however, suffer from various problems, which include low discharge voltages with rapid capacity fade (e.g., V2O5) and unclear speciation of the Al intercalation mechanism with insufficient capacity (e.g., graphite). The lack of ideal cathode materials is currently a major challenge for Al-ion batteries. Here, a high-capacity layered organic cathode composed of graphitic carbon nitride (g-C3N4) is developed for Al-ion batteries. Full cells constructed using g-C3N4 paired with an Al metal in an AlCl3/[EMIm]Cl electrolyte showed an open-circuit potential of 1.9 V and a capacity of 90 mAh/g cycled at 0.1 C. This battery also exhibits a stable capacity of 75 mAh/g cycled at 0.2 C in a long-term test (500 cycles). The data show that the layered porous structure of the organic cathode material facilitates a reversible deintercalation of [AlCl4]− anions, substituting them for Cl– in a more oxidized form of the g-C3N4. The data further illustrate that the anion shuttle is associated with a conversion between N and N+· states at the tertiary N­(C)3 positions of the g-C3N4 structure.