Electrochemical Performance of Citric Acid-Modified α-MnO2 Cathode for Aqueous Zinc-Ion Batteries

Aqueous zinc-ion batteries (AZIBs) are considered promising for large-scale energy storage owing to their intrinsic safety, low cost, and environmental friendliness. α-MnO2, with its unique 2 × 2 tunnel structure, is regarded as one of the most promising cathode materials for zinc-ion batteries; however, its cycling stability is poor due to the disproportionation and dissolution of Mn3+ and structural collapse. In this work, α-MnO2 was prepared by a hydrothermal method with citric acid introduced into the precursor solution, yielding a MnO2@CA cathode material. Results show that the citric acid modification does not change the main crystal structure of α-MnO2, and SEM images reveal that both the pristine and modified samples present nanorod morphology. Electrochemical tests demonstrate that after 100 cycles at 0.5 A·g−1, the MnO2@CA electrode still retains a discharge-specific capacity of about 200 mAh·g−1 with a capacity retention of about 90%, while the pristine α-MnO2 electrode decays to about 75 mAh·g−1. At 0.1 and 0.2 A·g−1, MnO2@CA delivers higher specific capacities than pristine α-MnO2, and it also shows better capacity recovery after the current density returns to 0.1 A·g−1. At high rates, the rate performance is somewhat limited by solid-state diffusion/concentration polarization after low-rate activation, while the long-term cycling stability is significantly improved. Kinetics analysis indicates that the electrode process is dominated by diffusion control, while the capacitive contribution increases monotonically with scan rate (from about 7% to 19%) but remains minor compared with the diffusion contribution. Citric acid modification is proposed to stabilize Mn species through chelation, thereby suppressing Mn dissolution, and alleviating structural stress, thereby effectively enhancing the cycling stability and low-rate discharge capacity of the α-MnO2 cathode.