Effect of magnesium compounds on crystal structure, morphology, and electrochemical properties of LiFePO4/C

The mechanism of magnesium compounds on the crystal structure, morphology, and electrochemical properties of LiFePO4 has been investigated using magnesium oxide, magnesium sulfate, and magnesium hydrogen phosphate as dopants. The shrinkage of crystal cell and lattice spacing implies that Mg2+ is successfully doped into the lattice of LiFePO4, since magnesium ions occupy less space than ferrous ions or lithium ions. Meanwhile, the primary particle size of LiFePO4 tends to reduce when high-content magnesium is doped, which is ascribed to cell shrinkage as well as the reduction of surface energy. In addition, agglomeration of crystal fine particles is observed in the SEM images. The discharge capacity of 1%MgHPO4-LFP increases as the proceeding of the charging/discharging cycle; i.e., the retention rate of this sample exceeds 100% (100.96%) at 0.5 C after 30 cycles. Owing to the substitution of S for O sites, the diffusion channel of Li+ expands while the total number of Fe-Li reverse site defects decreases, which lowers the capacity attenuation of 10%MgSO4-LFP under high-rate conditions (compared to 10%MgO-LFP). The discharge capacity of 10%MgSO4-LFP is 88.7 mAh/g, which is higher than 10%MgO-LFP (75.06 mAh/g).