Study On Coating Methods Of Cathode Material-LiNi_0.8Co_0.15Al_0.05O₂ For Lithium-ion Batteries

Ni-rich cathode material LiNi_0.8Co_0.15Al_0.05O₂ has been widely considered as one of the most promising cathode materials for lithium-ion batteries.It is widely applied as power source of electric vehicles for its high capacity and low cost.However several problems still exist.The existence of abundant Ni3+ and Ni4+ will cause the insability of chemical characteristics of cathode materials.Capacity fades rapidly under high voltage and elevated temperature.Besides,side reactions between electrode and electrolyte will be aggravated due to the existence of Ni4+,which will facilitate formation of solid electrolyte interface?SEI?layer.Solid electrolyte interface?SEI?layer causes the increase of impedance and attenuation of cycling performance.Furthermore,LiOH impurities on the surface of cathode material LiNi_0.8Co_0.15Al_0.05O₂ will react with LiPF6 and yield HF which will dissolve metal ions in cathode materials.Present study indicated that coating on the surface of Ni-rich cathode material LiNi_0.8Co_0.15Al_0.05O₂ can solve the problem of thermal and cycling instability.These coating materials can serve as physical barriers which can protect cathode material form HF erosion and solve the problem of dissolution of metal ions.Stable interface environment can restrain the side reaction between electrode and electrolyte.Previous study indicated that metal oxides,metal fluorides and metal phosphates can serve as promising coating materials.In this paper,we prepared LiNi_0.8Co_0.15Al_0.05O₂ cathode material with uniform and spherical morphology and coated metal oxides on it.We chose Al2O3 and TiO₂ as coating materials and chose wet method?sol-gel?method and ALD technology as coating methods.Conventional wet method selected aluminum isopropoxide as raw material for coating.We obtained three different Al2O3-coated LiNi_0.8Co_0.15Al_0.05O₂ materials with different amounts of Al2O3 coaling.Uncoated and Al2O3-coated LiNi_0.8Co_0.15Al_0.05O₂ materials were conducted by XRD,XPS,SEM and TEM test.The electrochemical performances test was also carried out After Al2O3 coating,there were no changes in the structure of cathode material and no impurities were observed.A thin coating layer was visually observed on the surface of LiNi_0.8Co_0.15Al_0.05O₂ particles,which proves that Al2O3 coating was uniform and successful.The 1.0 wt%Al2O3-coated sample showed the optimal electrochemical performance at 1C rate,which revealed the capacity retention of 71.7%at 25? and 70.1%at 55? after 100 cycles.Besides,this sample also revealed best rate capability.ALD technology selected TiCl₄ as raw material for coating.We obtained three different TiO₂-coated LiNi_0.8Co_0.15Al_0.05O₂ materials with different thickness of TiO₂ coating by controlling ALD growth cycles.Uncoated and TiO₂-coated LiNi_0.8Co_0.15Al_0.05O₂ materials were conducted by XRD,XPS,SEM and TEM test.The electrochemical performances test was also carried out.After TiO₂ coating,there were no changes in the structure of cathode material and no impurities were observed.An ultrathin and dense coating layer was visually observed on the surface of LiNi_0.8Co_0.15Al_0.05O₂ particles,which proves that TiO₂ coating was uniform and successful.The TiO₂-coated sample with 138 ALD cycles and 5nm thickness revealed the optimal electrochemical performance at 1C rate,which revealed the capacity retention of 90.21%at 25? and 73.21%at 55? after 100 cycles.Besides,this sample also revealed best rate capability.