Surface Modification And Properties Of Li-and Mn-Rich Cathode Materials

With the rapid development of new energy automobiles,there is an increasing demand for lithium ion batteries with high energy density.Li-and Mn-rich cathode materials with large specific capacity(>250 mAh·g-1)are supposed to achieve lithium ion batteries with high energy density(>350 Wh·kg-1).The high capacity of Li-and Mn-rich cathode materials benefits from the lattice oxygen redox reaction induced by the electrochemical activation of the Li2MnO3 component.However,the activation of the Li2MnO3 phase and oxygen redox reaction lead to lattice oxygen release and structure transformation,which cause a series of problems such as low initial columbic efficiency,poor rate capability,serious voltage and capacity degradation after subsequent cycles.Surface modifications including surface coating,surface doping and surface chemical treatment can suppress the surface oxygen loss and structure transformation by mitigating electrode/electrolyte interface ride reaction and enhancing surface structural stability.In the thesis,three surface modified strategies are adopted to improve the electrochemical performance of Li-and Mn-rich cathode materials.The main details are as follows:(1)A carbon nanotubes(CNTs)modified Li-and Mn-rich cathode material Li1.184Mn0.516Ni0.15Co0.15O2(LMR)is synthesized by a new strategy using these technologies of compressed air crush,high pressure micro-fluidization dispersion and spray dehydration.Compared to LMR with commercial CNTs conductive additives,the CNTs conductive network is uniformly distributed on the surface of LMR secondary spherical particles and exists between primary LMR particles,which better effectively improves the electron conductivity of LMR particles.The electrochemical performance tests show that the specific discharge capacity of CNTs modified LMR is 141.4 mAh·g-1 at 5C rate,by contrast,that LMR with CNTs conductive additives is 110.7 mAhVg-1.After 100 cycles at 1C,the discharge capacity of the CNTs modified LMR is 160.2 mAh·g-1,compared to 133.7 mAh·g-1 of LMR with CNTs conductive additives.According to electrochemical impedance spectroscopy(EIS)analysis before and after 100 cycles,the charge transfer between electrode and electrolyte is accelerated due to the dense CNTs network on the surface of LMR,which mitigate serious polarization on the surface of LMR during cycling.(2)A graphene oxide(GO)wrapped Li1.184Mn0.516Ni0.15Co0.15O2(LMR)cathode materials is achieved by high pressure micro-fluidization dispersion and spray dehydration.Then the GO wrapped LMR is adopted by ascorbic acid and air heat treatment,which promote the growth of graphene and spinel structure coating layer on the surface of LMR.According to a serious of material and electrochemical characterization methods,the ultra-high conductivity of graphene promotes surface electron transfer,and the three-dimensional pores of the spinel phase interlayer accelerate Li+ diffusion.As a result,the surface-modified LMR cathode exhibits a high specific discharge capacity of 154.3 mAh·g-1 at 5C rate;by contrast,that of the pristine LMR cathode is 105.6 mAh·g-1 at the same condition.After 200 cycles at 1C rate,the surface-modified LMR cathode has an excellent cycle stability with a capacity retention of 84.2%compared to the pristine LMR(77.0%).In addition,the structural evolution of the surface-modified LMR cathode material was studied by in-situ synchrotron X-ray diffraction analysis during the charge-discharge process,illustrating that the surface modification has a significant effect on mitigating oxygen release by enhancing the surface structural stability.(3)A microsize Li1.18Mn0.55Ni0.18Co0.09O2 is jet crushed by high-pressure air flow to obtain submicrosize sample with no change in composition and structure.The physicochemical and electrochemical property of microsize sample and submicrosize sample are studied.The results show that submicrosize sample possesses a higher rate capability(>1C)due to shorter Li+diffusion pathway within the sample particles,however,the increased side reactions with the electrolyte leads to poorer cycling performance.A cheap and easily available cerium oxide(CeO2)is used to coat the surface of the microsize and submicrosize sample,which can improve the reversible capacity and cycling performance.On this basis,a comprehensive engineering strategy combined microsize and submicrosize particles blending and CeO2 coating is proposed to enhance the electrochemical performance of Li1.18Mn0.55Ni0.18Co0.09O2.The submicrosize particles are blended into the large gaps left by the accumulation of microsize spherical particles,which contribute additional capacity due to the whole tap density elevation.As a result,the reversible discharge capacity of the modified sample is 191.8 mAh·g-1 at 1C and then the capacity retention remains 82.8%after 200 cycles.The analyses of surface composition illustrate that CeO2 coating layer increase the stability during cycling by suppressing the electrode/electrolyte side reactions.