Study On Surface Modification Of LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Material For Li-ion Batteries

Nickel-rich LiNi_0.6Co_0.2Mn_0.2O₂ has been considered as one of the most promising cathode materials due to its high discharge capacity.Nevertheless,its cycling performance is very poor,particularly at high voltages,because of the irreversible structural change during cycling.Another critical issue about LiNi_0.6Co_0.2Mn_0.2O₂ is the poor storage property,which is mainly caused by the slow reduction of Ni3+ to Ni2+ and the formation of Li2CO3/LiOH impurities stemmed from the reaction between air and synthetic residual Li2O/LiOH on particle surface,thus leading to the capacity loss of host material after storage.What's more,such impurities may cause gas evolution and safety issues during battery operation.To resolve these problems,surface coating via wet-chemical method is used to improve the electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ in this study.Structural-stable phosphate LiTi₂?PO₄?₃,CePO₄ and Co₃?PO₄?₂ are firstly used as coating materials to investigate the appropriate coating conditions of LiNi_0.6Co_0.2Mn_0.2O₂.A moderate amount of Co₃?PO₄?₂ can improve both the cycling stability and the initial discharge capacity of LiNi_0.6Co_0.2Mn_0.2O₂ when coated in ethonal.LiNi_0.6Co_0.2Mn_0.2O₂ is coated by Co₃O₄,MgO and Mn₂O₃ in ethanol,respectively.By comparing their initial discharge capacities and cycling stability,it is found that the electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ at 2.8-4.3 V can be improved most effectively by Co₃O₄ coating.A uniform Co₃O₄ layer can be coated on the surface of LiNi_0.6Co_0.2Mn_0.2O₂ via a facile polyvinyl pyrrolidone?PVP?-assisted wet coating process.Its highly continuous and compact coating layer is confirmed by scanning electron microscopy?SEM?and transmission electron microscopy?TEM?.1 wt.% Co₃O₄-coated LiNi_0.6Co_0.2Mn_0.2O₂ shows improved electrochemical performance at both 2.8-4.3 V and 2.8-4.6 V.Cyclic voltammetry?CV?results indicate that 1 wt.% Co₃O₄-coated LiNi_0.6Co_0.2Mn_0.2O₂ exhibits lower polarization and higher reversibility of electrode reaction.The storage property of LiNi_0.6Co_0.2Mn_0.2O₂ is also ameliorated by Co₃O₄ coating.After storing in air for three months,the initial discharge capacities of pristine and 1 wt.%Co₃O₄-coated LiNi_0.6Co_0.2Mn_0.2O₂ decrease from 206.4 mAh g-1 and 209.5 mAh g-1 to177.8 mAh g-1 and 198.6 mAh g-1 at 0.1 C?18 mA g-1?,respectively.Electrochemicalimpedance spectroscopy?EIS?results reveal that the charge transfer resistance of coated sample after storage is much smaller than that of stored pristine sample.The improved storage property also benefits from Co₃O₄ coating layer which can prevent the formation of electrochemical inactive Li2CO3/LiOH impurities.