The Study On Preparation Process And Performance Of LiNi_0.85-xCo_0.15Al_xO₂ Cathode Materials For Lithium Ion Batteries

Lithium-ion batteries are green and rechargeable secondary power.They are widely used in portable electronics,electric vehicle（EVs）and hybrid electric vehicles（HEVs）.The cathode material is a key factor affecting the energy density of lithium-ion batteries.LiNi_0.8Co_0.15Al_0.05O₂ is considered to be a promising cathode material as its high discharge capacity and high security.However,LiNi_0.8Co_0.15Al_0.05O₂ has some shortcomings,such as the complexity of synthesis methods and the instability of the electrochemical performance.Based on this,preparation conditions are optimized and the doping amount of Al are studied in this dissertation to enhance the electrochemical properties of LiNi_0.85-xCo_0.15Al_xO₂.First,the preparation conditions of coprecipitation reactions,which have impact on the particle size and morphology,are investigated and the optimum coprecipitation reaction conditions are found out.The electrochemical performances of a cathode material have high relation with the particle size,morphology and tap density of the precursor,which are affected by the stirring speed,reaction temperature and volume ratio of complexing agent and alkali solution in the coprecipitation reaction.The influences of the experiment parameters mentioned aboved and the electrochemical performances of the as-prepared cathode materials are investigated.The optimized preparation conditions of precursor materials are as follows:the coprecipitation temperature is 60 ℃,the volume ratio of complexing agent and alkali solution is 1:4,the stirring speed is 400 revolutions per minute and the coprecipitation time is more than 27 h.Second,the impacts of the doping amount of Al to the electrochemical performance of LiNi_0.85-xCo_0.15Al_xO₂ are studied.Based on the optimized preparation conditions mentioned above,Ni_0.85-xCo_0.15Al_x（OH）₂（x=0.02,0.035,0.05,0.06）precursors are prepared and after calcined,LiNi_0.85-xCo_0.15Al_xO₂ cathode material are synthesized.The uniform particle distribution and the largest tap density of the LiNi_0.85-xCo_0.15Al_xO₂ cathode material can be obtained when x=0.035.There is no impurity peak and good layered structure can be found in XRD pattern.The first discharge capacity is 187 m Ah g-1 at 0.1C,and the capacity retention of the cathode material is excellent.The influence of the particle size,morphology and tap density of Ni_0.85-xCo_0.15Al_x（OH）₂ precursor on the electrochemical properties of the as-prepared cathode material are studied.The LiNi_0.85-xCo_0.15Al_xO₂ cathode material with the excellent electrochemical performances are obtained.In a word,the preparation conditions of the coprecipitation precursor and the doping amount of Al are studied in this dissertation.The Ni_0.85-xCo_0.15Al_x（OH）₂ precursor with centralized particle size distribution and high tap density is obtained.Moreover,further study shows that LiNi_0.85-xCo_0.15Al_xO₂ cathode material has the better electrochemical performances when x=0.035.