Thermal and Structural Behavior of Surface-Modified Li[Ni0.7Co0.2Mn0.1]O2 As a Positive Electrode for Rechargeable Lithium Batteries

Lithium secondary battery is promising as a power backup for energy storage system. In this respect, studies towards improvement in performances of electrode are being progressed to assure long cycling performances, thermal stability, safety, and so on. In particular, positive electrode materials are need to be further investigated to accomplish the above- mentioned properties. Several kinds of 4 V class positive electrode materials are commercially available. Among them, LiCoO₂ is one of the most common electrode materials in rechargeable lithium batteries. Although the LiCoO₂ has many advantages, this material has poor capacity retention because of dissolution of Co and structural instability at deeply charged state. To improve the nature of active material, surface modification is effective to overcome those limited properties of active materials. Actually, Metal oxides as coating materials have been extensively studied. In this study, Metal phosphate coatings are applied on the surface of Li[Ni_0.7Co_0.2Mn_0.1]O₂. We, here, report the resulting structural, electrochemical and thermal properties of the surface-modified Li[Ni_0.7Co_0.2Mn_0.1]O₂.

H₃PO₄ and metal salts were selected as starting materials for surface modification of Li[Ni_0.7Co_0.2 Mn_0.1]O₂. A solution anhydrous ethanol containing metal phosphates was stirred at 30 ^oC for 5 hours. Then, active materials were added into the solution. And the solution was evaporized at 80 ^oC in air and the resulting precipitates were then heated at 500  ^oC in air. Also, the coated powders were characterized by XRD, SEM, and HR-TEM. Electrochemical properties of coated powders were examined by galvanostatic cycle test and electrochemical impedance spectroscopy.

Metal phosphates coatings shows uniform coating layers, as observed by TEM. TG and DSC analyses were conducted for the materials. Electrochemical test with half cells in voltage range of 3 - 4.3 V at 25 ^oC indicates that coated materials have better capacity retention and coulombic efficiency, rate capability and thermal properties. Thin coating layers were effective in improvement of the electrochemical, structural and thermal properties. Also, metal phosphates coatings give decrease in the residual lithium content and byproduct on the surfaces of particles, as confirmed by ToF-SIMS. And the materials were analyzed into XPS and ToF-SIMS to confirm surface byproducts of the materials itself at high temperature. Details will be mentioned at the conference site.