Research On Systhesis And Electrochemical Performance Of Manganese-based Cathode Materials For Lithium Ion Batteries

The electrochemical performance,service life and costs of lithium ion batteries largely depend on the cathode materials.Among commercial cathode materials,LiCoO2 has good electrochemical performance but high cost and toxicity of Co restrict its further application.As for spinel LiMn2O4,it has cost advantage while there are still many problems to be solved such as Jahn-Teller effect and manganese ion dissolution under overdischarging and at high temperature.LiFePO4 has good cycling stability while its theoretical capacity and energy density cann't fully meet needs for further development.LiNixCoyMn1-x-yO2 has high energy density but its thermal stability needs to be further improved.Many methods were explored to solve the problems existing in the current cathode materials,such as doping,coating and so on.In addition,new synthesis methods and new materials have been proposed.In this work,manganese-based cathode materials were synthesized by using layered double hydroxides as precursors,and their electrochemical performances and Li storage mechanisms were investigated and explored.The main research results are summarized as follows.1.We have studied the electrochemical performance and lithium storage mechanisms of Li-Mn-Al-O.MnAl layered double hydroxide was synthesized as a precursor to prepare Li-Mn-Al-O cathode material which exhibits excellent electrochemical performance.Li-Mn-Al-O can deliver a discharge capacity of 160.4 mAh g-1 in the first cycle and maintain 101.2 mAh g-1 in the 100th cycle under the current density of 20 mA g-1.Even at 60?,Li-Mn-Al-O can achieve a high discharge capacity of 160.6 mAh g-1 in the first cycle and remain 113.7 mAh g-1 after 100 cycles at 200 mA g-1.In situ XRD technique was used to study the lithium storage mechanism in Li-Mn-Al-O and Li-Mn-O.Li-Mn-Al-O exhibits more stable structure and excellent electrochemical performance because the substitution of Mn3+by Al3+alleviates the Jahn-Teller distortion and dissolution of Mn2+ions.2.On the basis of MnAl-LDH,Ni element was introduced to synthesize MnNiAl trinary layered double hydroxides as precursors.By adjusting the Ni content(x=0.1,0.2,0.3,0.4,0.5)in the precursors,Li-Mn-Ni-Al-O shows different electrochemical performances.Li-Mn-Ni-Al-O(x=0.3)shows a good comprehensive electrochemical performance.At 20 mA g-1,it delivers a discharge capacity of 164.8 mAh g-1 with a high energy density of 529.1 Wh kg-1 in the first cycle.By widening the measured voltage range to 2.0-4.8 V(vs.Li+/Li),Li-Mn-Ni-Al-O(xx=0.3)can achieve a high discharge capacity of 158.8 mAh g-1 and the energy density of 553.3 Wh kg-1 in the first cycle at 200 mA g-1.3.Based on the study of Li-Mn-Al-O and Li-Mn-Ni-Al-O cathodes,we considered to introduce Fe or Ce to Li-Mn-Ni-Al-O with the attempt to improve the structural stability and electrochemical performances.As compared with Li-Mn-Ni-AI-O,the electrochemical performance of Li-Mn-Ni-Fe-Al-O is not optimized obviously while Li-Mn-Ni-Ce-Al-O shows good structural stability after introducing Ce element.Li-Mn-Ni-Ce-Al-O delivers a high discharge capacity of 184.2 mAh g-1 in the first cycle and maintains 126.7 mAh g-1 after 100 cycles in 2.0-4.8 V(vs.Li+/Li)at 20 mA g-1.In addition,the capacity and energy density of Li-Mn-Ni-Ce-Al-O could be improved by further optimizing the contents of elements.Our studies provide a new insight into the development of new manganese-based cathode materials for lithium ion batteries with high energy density and stability.