Study Of Preparation And Properties Of Nickel-rich Layered Cathode Materials For Lithium Ion Batteries

Lithium ion battery is a new generation as high-energy batteries with excellent performance,which has become one of the key projects of high technology development.The layered nickel-rich materials are receiving increasing attention as cathode materials for lithium ion batteries(LIBs)due to their high capacity and low cost.However,nickel-rich materials suffer from structural instability and poor thermal stability.The study was based on a sol-gel method to systematically explore the situation of material surface,internal structure transition,morphology change,and electrochemical properties.The electrochemical properties of the materials were improved by modification.In the first place,Li2CO3 surface phase is inevitably formed in the layered nickel-rich materials in air normally which is considered to be detrimental to electrochemical performance,because Li2CO3 decomposes as well as deteriorates Li+ions extraction/insertion kinetics.We successfully tailor Li2CO3 surface phase simply by controlling calcination time in air.The thickness of Li2CO3 layer is observed to decrease with increased calcination time,accompanied by the reduced Ni2+ at surface.More oxygen vacancies caused by more Ni2+ owing to the difficulty of oxidizing Ni2+ to Ni3+at the surface are proposed here to be responsible for the formation of more Li2CO3 for shorter calcination times.Li2CO3 surface phase is evidenced to favor not only suppressing the H2/H3 phase transition but alleviating the interaction of LiNio.9Mn0.1O2 with electrolyte,which therefore improves the cycling stability of LiNi0.9Mn0.1O2 at low rates.On the other hand,Li2CO3 formed in Ni-rich LiNi0.9Mn0.1O2 surface is disadvantage to Li+ ions extraction/insertion kinetics as well as leads to poor rate capacity.In the second place,based on above study,the thick Li2CO3 on the material surface,which is beneficial to the cycle stability of the materials,but in fect,Li2CO3 is not in favor of Li+ ions extraction/insertion kinetics.Therefore,the precursor was heat-treatmented in oxygen atmosphere,inhibiting thick Li2CO3 generation,and comparing with the precursor heat-treatmented in air.The structural and morphological features of as prepared LiNi0.9Mn0.1O2 were investigated with XRD,SEM,XPS,FT-IR and TEM.The results indicate lower valence Ni ions existed in LiNi0.9Mn0.1O2 heat-treated under air atmosphere.Oxygen vacancies leads to serious cation mixing and formation of Li2CO3 on the surface of LiNi0.9Mn0.1O2 particles.This undesired phenomenon does harm to electrochemical performance because of the huge charge transfer resistance appeared and thick SEI film formation in the process of electrochemical reaction.However,the presence of oxygen in the atmosphere effectively makes cation mixing and formation of Li2CO3 suppressed.LiNi0.9Mn0.1O2 synthesized under oxygen atmosphere can improve the rate capacity as well as the cycling performance measured at the rate of 5C.Cyclability became worse at the high rate due to poor Li+ ions kinetics led by thick SEI film when LiNi0.9Mn0.1O2 synthesized under air atmosphere.Finally,Three samples of LiNi0.8Co0.lMn0.1O2,Al doping,and Al&Mg co-substitution were prepared.According to the XRD data,it was successfully doped.After the doping of the metal,the lattice a axis decreased and the c axis increased.Compared with LiNi0.8Co0.1Mn0.1O2,Ni2+ ions in Li+ site accounted for a decrease for Al&Mg co-substitution.XPS analysis showed that the Ni2+ on the surface of the material increased after doping.Both Cls and EIS analysis showed that the Li2CO3 on the surface of all samples was similar and less,which had little effect on electrochemical performance.Through co-substitution of Al&Mg,as resulted material enhanced the capacity,and the cycling stability is greatly improved under different conditions.