Preparation And Modification Of Li-rich Mn-based LiLi_0.2Ni_0.2Mn_0.6O₂Cathode Materials For Li-ion Batteries

Abstract:Compared with the conventional cathode materials for lithium-ion batteries, the Li-rich Mn-based LiLi0.2Ni0.2Mn0.6O2is wildly used for its high capacity, low cost, low pollution and excellent safety. Despite of all these advantages, there are several shortcomings which restrict the further development of the material, such as the high irreversible capacity loss of the first cycle, low stability under high voltage and insufficient rate capability.In this thesis, LiLi0.2Ni0.2Mn0.6O2was synthesized by sol-gel method using corresponding metal acetates and citric as the starting materials. The optimized synthetic conditions were found as-follow:calcination temperature900℃, pH value of the sol5.5, transition metal concentration0.5mol·L-1, molar ratio of citric to transition metal0.5. XRD reveals that the as-prepared LiLi0.2Ni0.2Mn0.6O2has a well defined, layered structure which is indexed based on the α-NaFeO2structure with the characteristic diffraction peaks of Li2MnO3. The first charge and discharge specific capacity of the material are260.8and198.0mAh·g-1at0.1C, respectively. The charge-discharge and cyclic voltammetry test results show that Mn3+and Mn4+participate in electrochemical reactions in the following cycles after the activation of Li2MnO3in the first cycle.LiLi0.2Ni(0.2-0.5x)Mn(0.6-0.5x)CrxO2(0≤x≤0.20) was also prepared by sol-gel method. The XRD result shows that Cr-doping does not change the structure of the material but the lattice parameters are increased. SEM morphology shows that doping with appropriate amount of Cr can reduce the agglomeration of particles and thus favor the formation of uniform particles. The cyclic voltammetry, electrochemical impedance spectroscopy(EIS) and charge-discharge test indicate that doping with appropriate amount of Cr can reduce the voltage difference between the oxidation peak and reduction peak, lower the resistance and improve the electrochemical performance. Among the synthesized samples, the one doping with Cr at x=0.04exhibits the best electrochemical performance, which delivers the first cycle discharge specific capacity of214.9,187.2 and153.9mAh·g-1at0.2C,0.5C and1C, respectively.LiLi0.2Ni0.2Mn0.6O2was coated with CaF2in the alcohol solution. XRD result reveals that the structure of the material remains the same. The charge-discharge tests of the samples coated with different amount of CaF2show that the first cycle coulombic efficiency, rate capability and cycling performance are improved although the first cycle discharge capacity is slightly lowered. The sample coated with1wt.%CaF2exhibits the best electrochemical performance, with the first cycle coulombic efficiency of78.1%and the rate retention of the capacity89.0%after100cycles at1C. The EIS test of the samples after100cycles at1C shows that the electrochemical resistance is reduced and their electrochemical performance is improved.