Calcium cation enhanced cathode/electrolyte interface property of Li₂FeSiO₄/C cathode for lithium-ion batteries with long-cycling life

Currently, the cycle performance at low rate is one of the most critical factor for realizing practical applications of Li₂FeSiO₄/C as a cathode of the lithium-ion batteries. To meet this challenge, calcium (Ca)-doped Li₂FeSiO₄/C is prepared by using the sol-gel method with soluble Li, Fe, Si and Ca sources. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy measurements are carried out to determine the crystal structures, morphologies, particle sizes and chemical valence states of the resulting products. Rietveld refinement confirms that Ca-doped Li₂FeSiO₄ has a monoclinic P2₁/n structure and that a Ca cation occupies the Fe site in the Li₂FeSiO₄ lattice. The grain size of Ca-doped Li₂FeSiO₄ is approximately 20 nm and the nanoparticles are interconnected tightly with amorphous carbon layer. As a cathode material for the lithium-ion batteries, Li₂Fe_0.97Ca_0.03SiO₄/C delivers a high discharge capacity of 186 mAh g⁻¹ at a 0.5 C rate. Its capacity retention after the 100th cycle reaches 87%, which increases by 25 percentage points compared with Li₂FeSiO₄/C. The Li₂Fe_0.97Ca_0.03SiO₄/C cathode exhibits good rate performance, with corresponding discharge capacities of 170, 157, 144 and 117 mAh g⁻¹ at 1 C, 2 C, 5 C and 10 C rates, respectively. In summary, the improvement of the electrochemical performance can be attributed to a coefficient of the strengthened crystal structure stability during Li⁺ deintercalation-intercalation and restrained side reactions between electrode and electrolyte.