Study On Modification Of Ni-rich LiNi_0.8Co_0.1Mn_0.1O₂ Cathode Materials For Lithium Ion Battery

Ni-rich layered LiNixCoyMnzO2(NCM,x?0.8)oxides have been attractive cathode materials of high-energy lithium-ion batteries for large-scale applications.However,pristine NCM cathode materials with high Ni contents encounter drastic side reactions at the particle surface and inherent phase transitions in the layered structure during cyclic lithiations/delithiations.Its thermal stability is poor and the surface is liable to generate residual alkali,leading to poor cycling stabilities upon prolonged cycles.In view of these problems,from the perspectives of surface coating,internal doping and structural regulation,we studied coating and doping methods to modify LiNi0.8Co0.1Mn0.1O2(NCM811),which was synthesized by sol-gel method.In addition,a co-precipitation method was explored to make synthesis of NCM811 more convenient and environmentally friendly The conclusions of this work were listed as following:1.An uniform Li2MnO3 coating layer was synthesized successfully on the surface of NCM811.Testing by neutralization titration showed that the residual alkalis(LiOH and Li2CO3)on the surface of NCM811 coated with Li2MnO3 were significantly reduced.What's more,in-situ X-ray diffraction(XRD)measurements have been carried out to probe structural configurations of the Ni-rich LiNi0.8Co0.1Mn0.1O2(NCM811)cathode material during charge/discharge processes.The operando XRD results reveal undesired structural irreversibility of NCM811 cathode material in the first cycle.In the subsequent electrochemical cycles enhanced structural stability can be implemented for reversible phase transformations via a Li-rich Li2MnO3(LMO)coating.Over the voltage range of 2.7-4.3 V in charge/discharge,the optimized NCM811@LMO-3%cathode material shows high capacity retentions of 92.9%at 0.1 C after 100 cycles and 80.4%at 1 C after 500 cycles,significantly superior to that of the bare NCM811 material(73.8%and 63.1%).2.The NCM811 was doped with Li2MnO3 by the sol-gel method.Through Rietveld refinement,it was found that the Li2MnO3 doped NCM811 had a Li+/Ni2+cation mixing ratio of 1.87%,which was much lower than the 2.51%of pure NCM811.Li2MnO3 can effectively improve structural order of NCM811 and reduce cation mixing effect.In DSC test results,the main exothermic peak of pure NCM811 is located at 245.8? and the reaction heat is 1060 J g-1,while the main exothermic peak of NCM811 doped by Li2MnO3 moves to 262.1? and the reaction heat is significantly reduced to 657.4 J g-1,indicating that Li2MnO3 can improve thermal stability of NCM811,and thus improve safety performance of the battery.3.In the process of preparing NCM811 by co-precipitation method,an environmentally friendly Na2CO3 was used instead of NaOH and NH3·H2O,which are used in traditional co-precipitation method,to explore preparation process of NCM811.The testing results show that the optimal molar amount of Na2CO3 is twice that of NCM811 and the preferred precipitation temperature is 80?.The best conditions in synthesis process are calcination at 875? for 5 h in an oxygen atmosphere.The optimal molar ratio of lithium source to metal ions is 1:1.The structure of NCM811 prepared by this method is verified to be suitable,and the electrochemical performance is measured showing good results at 0.1 C and 1 C rates in the voltage range of 2.7-4.3 V.This work provides guidance for preparation of NCM811 material by a new co-precipitation method.