Nanoflower-like K-birnessite cathode material for high-capacity aqueous Zn-ion battery

Aqueous zinc ion batteries (AZIBs) have become a research hotspot due to their advantages of low cost, high safety, and environmental friendliness. K-Birnessite (K_xMnO₂) has been proved to be a candidate cathode material. However, the low conductivity and capacity degradation issues of δ-MnO₂ limit its application. In the study, K_xMnO₂ (x = 0.27 and 0.31) with a nanoflower structure to improve the specific capacity has been designed, and capacity deterioration related to the evolution of Zn₄SO₄(OH)₆·xH₂O was analyzed. The capacity of K0.27MnO2 and K0.31MnO2 were 401.8 and 412.4 mAh g⁻¹ at 0.1 A g⁻¹ in the initial cycle due to the large specific surface area provided by the nanoflower structure. After 100 cycles, the specific capacity of the K_0.27MnO₂ electrode was 156.8 mAh g⁻¹ at 1.0 A g⁻¹, with a capacity retention rate close to 80%. During cycling, Zn₄SO₄(OH)₆·xH₂O was formed on the surface of the K_xMnO₂ cathode and transformed from a thin slice to a cracked block, leading to slow ion transport. This work provides a perspective for high-performance cathode design in AZIBs.