Surface modification of a cobalt-free layered Li[Li_0.2Fe_0.1Ni_0.15Mn_0.55]O₂ oxide with the FePO₄/Li₃PO₄ composite as the cathode for lithium-ion batteries

A low-cost, layered Li[Li_0.2Fe_0.1Ni_0.15Mn_0.55]O₂ oxide is successfully coated with the FePO₄/Li₃PO₄ composite by an aqueous solution method to achieve high electrochemical performance. X-ray diffraction (XRD) patterns indicate that the modified sample is a hexagonal phase with a minor crystalline Li₃PO₄ phase inside. Compared with the pristine sample, the modified ones show no change in morphology characterized by scanning electron microscopy (SEM) analysis. However, a uniform coating layer of the FePO₄/Li₃PO₄ composite can be observed clearly in the transmission electron microscopy (TEM) images. By electrochemical characterization, the composite coating layer is proved to be beneficial for improving the reversible capacity and cycling stability of the modified sample (a higher reversible discharge capacity of 192 mA h g^-1 after 50 cycles with a 3 wt% coating amount). Surprisingly, the high-rate capability is also observed to be improved with a 3 wt% coating amount (125.3 mA h g^-1 after 100 cycles at 10 C). Furthermore, the voltage decay phenomenon during cycling is slowed down greatly and thus phase transformation is suppressed by the composite coating layer. These results are attributed to the suppression of the bulk material from direct exposure to the electrolyte by the amorphous FePO₄ coating component and the good Li⁺ transport through the Li₃PO₄ coating component. The prepared modified materials can meet the requirements of low cost and high performance in various applications for lithium-ion batteries.