Study On Synthesis, Modification, And Low Temperature Properties Of Li-Ni-Co-Mn-O Cathode Materials For Lithium Ion Batteries

The layered Ni-rich cathode material,LiNi_xCo_yMn_zO₂?NCM?,is regarded as the most promising cathode material for use in power batteries for electric vehicles due to its higher energy density and lower cost compared to the conventional LiCoO₂ material.However,Ni-rich NCM exhibits poor cyclic stability and the capacity decreases immediately,owing to the undesired side reactions of the interface between the electrode and electrolyte at high voltage during cycling,significantly limiting their commercial application.To address these problems,NCM523@SnO₂ and NCM622 cathode materials with high specific capacity and excellent cyclic stability are prepared via plasma-assisted milling.Then the microstructure and electrochemical performance of the NCM materials are studied,and the mechanisms of enhanced cyclic stability are revealed.Furthermore,NCM622 cathode material has been assembled into half-cells to obtain their electrochemical performance and kinetic characterization at different low temperature.The main conclusions are as followed:Firstly,NCM523@SnO₂ composites are successfully prepared by plasma-assisted ball milling,which has a microstructure that conductive SnO_2-x full of oxygen vacancy-rich particles are effectively distributed on the surface of NCM523 particles.As a result,the electrode consisting of milled NCM523 coated with 3 wt.%SnO₂?NCM523-3SnO₂?exhibits a high initial coulombic efficiency of 82.7%and a good capacity retention of 92.3%after 150cycles.It is demonstrated that the synergistic effect of high energy plasma and milling-induced oxygen vacancies in the SnO_2-x surface protection layers of the NCM523cathode enable greatly increased conductivity of the active materials and stable interfaces between the electrolyte and electrode.These factors are beneficial to provide a higher discharge capacity,superior high-rate capability and enhanced cyclic stability in the NCM523-3SnO₂ cathode.Secondly,after planetary ball milling mixture process and sintering at 800?,the as-prepared NCM622-800?-QM cathode with uniform and moderate particles exhibits high ICEs of 81.29%and first discharge specific capacity of 174.5 mAh·g^-1,and a good capacity retention of 98.1%after 300 cycles.The process of mixing the precursor of NCM622 with Li₂CO₃ via plasma-assisted milling can effectively reduce the high temperature sintering temperature to 780?,and the microstructure and excellent electrochemical performance of as-prepared NCM622-780?-PB cathode are similar to NCM622-800?-QM cathode.The XRD and SEM measurements of two kinds of NCM622 samples prove that they both have uniform and moderate particle size,high crystallinity and good layer structure,which are beneficial to provide a higher discharge capacity,superior high-rate capability and enhanced cyclic stability in the NCM622 cathode.Finally,half cells are assembled and electrochemically studied with NCM622-780?-PB as the cathode and EC/DEC and EC/PC/EMC as electrolytes.It is found that,both the first discharge capacity and the ICEs of the NCM622 cathode continue to decrease with the decrease of temperature.The charge and discharge test at 30?at a current rate of 0.5C is abnormal,but 0.2C-rate charge-discharge can be achieved at the same temperature.When the test temperature decreases from 30?to 20?,the R_f and R_ct of NCM622 cathode with EC/DEC electrolyte increase by 53 and 180 times,while the diffusion coefficient of Li⁺in the active material only decrease by 21 times,indicating that the polarization is mainly caused by the sharp increase of film resistance and charge transfer resistance at low temperature.Compared with EC/DEC electrolyte,NCM622 cathode with EC/PC/EMC electrolyte exhibits higher first discharge capacity and higher ICEs at low temperature because of its better ion conductivity and higher diffusion coefficient of Li⁺,but at 30?it shows lower higher first discharge capacity and lower ICEs than the NCM622 cathode with EC/DEC electrolyte due to its poor Li⁺transfer ability,which indicates that the optimization of suitable electrolyte system will be one of the important measures to enhance the low temperature performance of NCM622 cathode.