Core-Shell Structure in Li-Battery Application: Phase Control and Electrochemical Property

Abstract

Architectures based on core-shell structures have been employed in different approaches for materials design in order to tune physical characteristics and chemical function. In the case of batteries, using active cathode materials as core and non-active metal oxide shells is a prevailing strategy to decrease chemical degradation at electrode/electrolyte interfaces. The main challenge becomes to balance the need for high Li ion diffusion while enable electronic insulation to avoid secondary redox processes that degrade the electrolyte or induce the loss of the active transition metals into solution. Traditional synthetic methods based on high temperature treatments induce compositional inhomogeneity in core-shell structures due to excessive diffusion, resulting in difficulties to form uniform, yet ultrathin shells. Further, any synthetic strategy must enable the tuning of the chemical species in the surface inactive oxide shell is important to influence the protective function of the inner active oxide core. Herein, we report a new strategy to synthesize well-defined LiCoO2 nanocrystals with ultrathin shells based on Al. Through variations in Al loading and post-synthetic treatment, the specific phase and durability were tailored. The progression between amorphous Al2O3 and LiAlxCo1-xO2 with temperature was evaluated by XRD and EDX line-scan analysis. Soft X-ray was used to probe the changes in chemical bonding both at the surface and bulk of the material. The electrochemical properties were found to vary depending on these compositional parameters. Compared to bare LiCoO2 nanocrystals, amorphous Al2O3 led to higher capacity and average stability, with LiAlxCo1-xO2 providing much higher stability but lower capacity. A tailored compromise between these two end situations will be discussed.