Construction of hierarchical K0.7Mn0.7Mg0.3O2 microparticles as high capacity & long cycle life cathode materials for low-cost potassium-ion batteries

Abstract

Potassium-ion batteries (PIBs) are regarded as a promising energy storage appliance for large-scale applications due to their abundant K reserves and fast K+ conductivity in electrolyte. However, the commercial production of PIBs is greatly hampered owing to the lack of stable cathode materials with high structural stability to accommodate insertion/extraction of large K ions. Here, novel hierarchical K0.7Mn0.7Mg0.3O2 microparticles (denoted as H-K0.7Mn0.7Mg0.3O2) are designed and fabricated. When used as cathode materials for PIBs, the H-K0.7Mn0.7Mg0.3O2 exhibits a high reversible capacity (144.5 mAh g−1 at 20 mA g−1), fast rate capability (58.4 mAh g−1 at 400 mA g−1), and long cycling stability (a capacity retention of 82.5% after 400 cycles at 100 mA g−1). The storage mechanism and reaction kinetics are explored by a combination of ex situ X-ray diffraction analyses and electrochemical characterizations. The combination studies suggest that the superior performance of H-K0.7Mn0.7Mg0.3O2 can be attributed to the rational Mg doping and the unique hierarchical structure that leads to a single-phase reaction and fast K+ diffusion with low energy barriers during K+ insertion/extraction. Moreover, a K-ion full cell of H-K0.7Mn0.7Mg0.3O2//hard carbon demonstrates a promising energy density of 127.9 Wh kg−1 at 100 mA g−1 with a decent capacity retention of 75% over 100 cycles. These results suggest the potential application of this novel low-cost H-K0.7Mn0.7Mg0.3O2 as a high-performance cathode for PIBs.