Improvement of the electrochemical performance of an NCA positive-electrode material of lithium ion battery by forming an Al-rich surface layer

Abstract

A positive-electrode material of LiNi0.80Co0.15Al0.05O2 (NCA) having an Al-rich surface layer was synthesized, and its characteristics and electrochemical performance were investigated and compared with those of pristine NCA prepared by a conventional solid-state reaction. The Al-rich surface layer with its compositional gradient was formed by a wet-coating method using a fluidized bed coating technique and a subsequent heat treatment: fluidized NCA powder and a mist of 2-propanol solution containing an aluminium oxide sol were mixed in a chamber, followed by heat treatment at 600 °C in an oxygen atmosphere. For the electrochemical measurement, the positive electrode fabricated using the positive-electrode material was assembled into a 2032-type coin cell with lithium metal as an anode and LiClO4 in ethylene carbonate-diethyl carbonate as an electrolyte. The measurement revealed that the Al-rich surface layer effectively contributed to the improvement of charge-discharge cycle life performance; NCA with the Al-rich surface layer retained 96% of the initial capacity after 100 cycles, compared to the 90% retained by pristine NCA. For a better understanding of the improvement, the structure of the positive-electrode materials after 100 cycles was examined by scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM). High-angle annular dark-field signal images obtained from STEM showed no clear difference between these materials in terms of the growth of inactive rock-salt-like structure layer near the surface of the primary particles. On the other hand, low-magnification SEM observation revealed that the pristine NCA had widened microcracks among the primary particles in the secondary particles, while the NCA with the Al-rich surface layer did not. Therefore, the Al-rich surface layer improved the fracture strength of the secondary particles during the charge-discharge cycling, thus preventing the formation of isolated primary particles, which do not contribute to the charge-discharge behaviour.