Multifunctional AlPO4 coating for improving electrochemical properties of low-cost Li[Li0.2Fe0.1Ni0.15Mn0.55]O2 cathode materials for lithium-ion batteries.

Layered Li-rich, Fe- and Mn-based cathode material, Li[Li0.2Fe0.1Ni0.15Mn0.55]O2, has been successfully synthesized by a coprecipitation method and further modified with different coating amounts of AlPO4 (3, 5, and 7 wt %). The effects of AlPO4 coating on the structure, morphology and electrochemical properties of these materials are investigated systematically. XRD results show that the pristine sample is obtained with typical Li-rich layered structure and trace amount of Li3PO4 phase are observed for the coated samples. The morphology observations reveal that all the samples show spherical particles (3-4 μm in diameter) with hierarchical structure, composed of nanoplates and nanoparticles. XPS analysis confirms the existence of AlPO4 and Li3PO4 phases at the surface. The electrochemical performance results indicate that the sample coated with 5 wt % AlPO4 exhibits the highest reversible capacity (220.4 mA h g(-1) after 50 cycles at 0.1C), best cycling performance (capacity retention of 74.4% after 50 cycles at 0.1C) and rate capability (175.3 mA h g(-1) at 1C, and 120.2 mA h g(-1) at 10C after 100 cycles) among all the samples. Cycle voltammograms show good reversibility of the coated samples. EIS analysis reveals that charge transfer resistance after coating is much lower than that of the pristine sample. The excellent electrochemical performances can be attributed to the effects of multifunctional AlPO4 coating layer, including the suppression of surface side reaction and oxygen vacancies diffusion, the acceleration of lithium ions transport as well as the lower electrochemical resistance. Our research provides a new insight of surface modification on low-cost Li-rich material to achieve high energy as the next-generation cathode of lithium-ion batteries.