Influence of cerium doping on structure and electrochemical properties of LiNi0.5Mn1.5O4 cathode materials

Pristine LiNi_（0.5）Mn_（1.5）O_4 and cerium doped LiCe_xNi_（0.5–x）Mn_（1.5）O_4（x=0.005, 0.01, 0.02） cathode materials were synthesized by solid-state method. The effect of Ce doping content on structure and electrochemical properties of LiNi_（0.5）Mn_（1.5）O_4 cathode material was systematically investigated. The samples were characterized by X-ray diffraction（XRD）, Fourier transformation infrared spectrometer（FT-IR）, scanning electron microscopy（SEM）, electrochemical impedance spectroscopy（EIS）, cyclic voltammetry（CV） and constant-current charge/discharge tests. The results showed that Ce doping did not change the cubic spinel structure with Fd3m space group, but effectively restrained the formation of Li_xNi_（1–x）O impurity phase. Appropriate Ce doping（x=0.005） could decrease the extent of confusion between lithium ions and transition metal ions, increase the lattice parameter and Ni/Mn disordering degree（Mn^（3＋） content）. The synergic effects of the above factors led to the optimal electrochemical performance of LiCe_（0.005）Ni_（0.495）Mn_（1.5）O_4 sample. The discharge capacity at 10 C rate could reach 115.4 mAh/g, 94.82% of that at 0.2C rate, and the capacity retention rate after 100 cycles at 1C rate could reach 94.51%. However, heavier Ce doping had an adverse effect on the electrochemical properties, which might be due to the lower disordering degree and existence of more CeO_2 secondary phase.

Nanocomposite LiFePO_4·Li_3V_2(PO_4)_3/C synthesized by freeze-drying assisted sol-gel method and its magnetic and electrochemical properties

Nano-sized LiFePO_4·Li_3V_2（PO_4）_3/C was synthesized via a sol-gel route combining with freeze-drying. X-ray diffraction results show that this composite mainly consists of olivine Li Fe PO4 and monoclinic Li3 V2（PO4）3 phases with small amounts of V-doped LiFePO_4 and Fe-doped Li_3V_2（PO_4）_3. The magnetic properties of LiFePO_4·Li_3V_2（PO_4）_3/C are significantly different from LiFePO_4/C. Trace quantities of ferromagnetic impurities and Fe_2P are verified in LiFePO_4/C and LiFePO_4·Li_3V_2（PO_4）_3/C by magnetic tests, respectively. LiFePO_4·Li_3 V_2（PO_4）_3/C possesses relatively better rate capacities and cyclic stabilities, especially at high charge-discharge rates.The initial discharge capacities are 136.4 and 130.0 mA h g^（-1）,and the capacity retentions are more than 98% after 100 cycles at 2C and 5C, respectively, remarkably better than those of LiFePO_4/C. The excellent electrochemical performances are ascribed to the mutual doping of V^（3＋）and Fe^（2＋）, complementary advantages of LiFePO_4 and Li_3V_2（PO_4）_3 phases, the residual high-ordered carbon and Fe_2P with outstanding electric conductivity in the nanocomposite.