Cooling Induced Surface Reconstruction during Synthesis of High-Ni Layered Oxides
Transition metal layered oxides have been the dominant cathodes in lithium-ion batteries, and among them, high-Ni ones (LiNi x Mn y Co z O 2 ; x ≥ 0.7) with greatly boosted capacity and reduced cost are of particular interest for large-scale applications. The high Ni loading, on the other hand, raises the concern of surface instability and poor rate performance. The rational design of synthesis leading to layered LiNi 0.7 Mn 0.15 Co 0.15 O 2 with greatly enhanced rate capability is reflected, by implementing a quenching process alternative to the general slow cooling. In situ synchrotron X-ray diffraction, coupled with surface analysis, is applied to studies of the synthesis process, revealing cooling-induced surface reconstruction involving Li 2 CO 3 accumulation, formation of a Li-deficient layer and Ni reduction at the particle surface. The reconstruction process happens predominantly at high temperatures (above 350 °C) and is highly cooling-rate dependent, implying that surface reconstruction can be suppressed through synthetic control, i.e., quenching to improve the surface stability and rate performance of the synthesized materials. These findings may provide guidance to rational synthesis of high-Ni cathode materials.