Re-heating effect of Ni-rich cathode material on structure and electrochemical properties

• Re-heating effect of Ni-rich Li[Ni0.7Mn0.3]O2 cathode material is investigated.
• Re-heating is necessary for dehydration and for surface modification up to 700 °C.
• Lithium diffusion towards surface from particle is unavoidable during re-heating.
• Re-heating increases cation mixing and concentration of LiOH and Li2CO3.

The re-heating effect for Ni-rich Li[Ni0.7Mn0.3]O2 is investigated because the process is required in surface modification and removal of adhered water molecules. A representative binary Ni-rich Li[Ni0.7Mn0.3]O2 (in which cationic distribution in Li layers is not affected by heteroelements) is selected and synthesized via co-precipitation. The as-synthesized Ni-rich Li[Ni0.7Mn0.3]O2 is re-heated at 200 °C, 400 °C, and 600 °C, so that the resulting structural and electrochemical properties are compared by means of X-ray diffraction, transmission electron microscopy, time of flight-secondary ion spectroscopy, thermogravimetric analysis, high temperature X-ray diffraction, and electrochemical tests. Raising the re-heating temperature increases the occupancy of Ni2+ in Li layers and accelerates the aggregation of lithium-related compounds such as Li2CO3 and LiOH towards the particle surface. Among the several conditions tested, re-heating at 200 °C results in a negligible change in the crystal structure; specifically, Ni2+ occupation in Li layers, higher capacity with good reversibility upon cycling tests, better rate capability, and thermal properties. Therefore, re-heating of cathode active materials, in particular Ni-rich compositions, should be considered to stabilize both electrode performances and thermal properties.