Improved Electrochemical Performances and Thermal Stability of LiNi0.6Co0.2Mn0.2 by Li3PO4 coating

LiCoO₂ is the most commercialized in lithium ion batteries (LIBs) for portable electronics because of its high volumetric energy density compared to the other commercialized cathode materials. However, low specific capacity (145 mAh/g @ 4.3 V cut-off) hinders its application to emerging technologies, such as power tool and plug-in hybrid electric vehicles. As a result, layered Ni-rich LiNi_xCo_yMn_1‑x‑yO₂ materials have attracted much attention as promising alternative cathode materials because of their high capacity and relatively low cost. However, Ni-rich layered cathode materials suffer from structural instability by direct contact with electrolyte at high voltage (>4.3V), resulting in poor cyclability, rate capability and thermal instability. To solve these problems, a lot of researches have been carried out to coat the surface of layered cathode materials using oxide or phosphate, such as TiO₂, Al₂O₃, and Li₃PO₄.

Among these coating materials, Li₃PO₄ has attracted a great attention as coating material due to its high ionic conductivity and thermal stability due to strong P=O bonding. Recently, Zhao et al., reported on the Li₃PO₄ coating on LiFePO₄ cathode material, but they used Li₃PO₄ nano particle as a coating material, resulting in non-uniform coating. Also, Xiong et el, tried to coat the Li₃PO₄ on LiNi_0.8Co_0.1Mn_0.1O₂ cathode material using the reaction between (NH₄)₂HPO₄ and lithium residues, but the Li₃PO₄ coating layer is not confirmed.

In this study, we report on the synthesis of Li₃PO₄-coated LiNi_0.6Co_0.2Mn_0.2O₂ using a citric acid assisted sol-gel method in which nm-thick Li₃PO₄ layer was uniformly coated on the LiNi_0.6Co_0.2Mn_0.2O₂. It exhibited improved cyclability at high voltage (4.7V) and rate capability than those of bare LiNi_0.6Co_0.2Mn_0.2O₂. In addition, Li₃PO₄-coated LiNi_0.6Co_0.2Mn_0.2O₂ showed improved thermal stability at elevated temperature.

More details on the synthetic procedure, electrochemical and physical properties will be presented at the meeting.