Characterization And Electrochemical Properties Of Li₂FeSiO₄ Cathode Materials By Ti Doping And Molten Salt Method

Li₂FeSiO₄ has attracted tremendous attention because of its overwhelming advantages,such as high theoretical capacity,low cost of raw materials and environmental benignity.Unfortunately,the practical application of Li₂FeSiO₄ is hampered by its low intrinsic electronic conductivity and slow diffusion of lithium ions.Therefore,electrochemical properties of Li₂FeSiO₄ are studied from Ti⁴⁺doping and molten salt method in this paper.1 The Li_2-xTi_xFe_0.8Mn_0.2SiO₄/C?x=0,0.02,0.05?and Li₂Fe_0.8-xTi_xMn_0.2SiO₄/C?x=0,0.02,0.05?composites which Ti⁴⁺substitute Li sources and Fe sources are prepared by a solid state method and characterized by galvanostatic charge/discharge measurement.The results indicate that the discharge capacities of Li_1.98Ti_0.02Fe_0.8Mn_0.2SiO₄/C and Li_1.95Ti_0.05Fe_0.8Mn_0.2SiO₄/C are 169.1 and154.9 mAh·g-1,respectively,after 50 cycles,the discharge capacity retention are 42% and 30%.They are higher than the Li₂Fe_0.8Mn_0.2SiO₄/C sample with the capacity of 121.8 mAh·g-1and 28%,which is attributed to generated lithium vacancy.It can be propitious to the deintercalation of interior lithium ions.Among them,the performance of the Li_1.98Ti_0.02Fe_0.8Mn_0.2SiO₄/C is the best and the capacities of Li_1.95Ti_0.05Fe_0.8Mn_0.2SiO₄/C samples decrease due to the loss of Li source.When x=0.02 and x=0.05,the first discharge capacities of samples are significantly reduced to 104.7and 84.8mAh·g-1,resulting from the effect of impurities.2 The Li₂FeSiO₄ composites are synthesized by a molten salt method in three different molten salt systems.Effects of different molten salts on the structure and morphology of Li₂FeSiO₄ are examined by XRD,SEM.The results indicate that the Na NO₃+KNO₃ molten salts system with a relatively low melting point?228??can not synthesis the Li₂FeSiO₄samples.The NaCl+KCl,Na₂CO₃+K₂CO₃molten salts system with a relatively high melting point?500-700??can prepare the Li₂FeSiO₄,which can be indexed to the monoclinic unit cell P21/n and have no impurity phase.Because of the NaCl+KCl molten salts system takes advantage of cost,the Li₂FeSiO₄ samples are prepared at 700? for 5h when the mole ratio of Li:Fe:Si:?NaCl+KCl?is 2:1:1:4,which display loose aggregates and the surface of particles is smooth and clear.In addition,the effect of different carbon content on the electrochemical properties of Li₂FeSiO₄/C are further studied.The Li₂FeSiO₄/C with 13.5% carbon exhibits good electrochemical properties,which the discharge capacity is 175.9 mAh·g-1and the capacity retention is 80 % after 50 cycles.It is superior to the similar sample prepared by the solid state method.