Egg-shell structured LiCoO2 by Cu2+ substitution to Li+ sites via facile stirring in an aqueous copper(ii) nitrate solution†
For practical, high-energy lithium ion batteries, we introduce an egg-shell structured LiCoO2, enabling a credible performance with a high cut-off potential of 4.4 V, simply prepared by only stirring in 0.5 mM Cu(NO3)2 aqueous solution at room temperature without costly heat treatment. Through this, a very robust structure in which the Li+ ions in the LiCoO2 structure were substituted with Cu2+ is selectively synthesized on the surface of active material particles. The egg-shell structured LiCoO2 presents excellent cyclability and high coulombic efficiency even in a high potential range of 4.4 V (vs. Li/Li+) to allow a high specific capacity. Additionally, the surface-modified LiCoO2 reaches a high capacity at a fast discharge rate of 20C even after a high temperature cycling sequence. The intended surface modification is also carefully investigated by systematic analyses of STEM, SEM and XPS. Finally, the electrochemical performance of a graphite/surface-modified LiCoO2 full-cell exhibits an excellent capacity retention of nearly 90% over 1000 cycles. It is concluded that the surface modification method in which the LiCoO2 powder is added to a low-concentration aqueous solution of copper nitrate and stirred at room temperature is effective in improving the cycle life of LiCoO2 at high potentials.