Preparation and performances of novel Na2FeSiO4/C composite with more stable polymorph as cathode material of sodium-ion batteries

The sodium orthosilicates as the potential candidates of cathode materials in sodium-ion batteries capture tremendous interests because of the low costs, robustly crystal structure and high specific capacity. However, like lithium orthosilicates, the crystal structure, morphology and electrochemical properties of the polycrystalline sodium orthosilicates need to be properly designed so that sodium ion can be smoothly intercalated/deintercalated. Herein, a novel orthorhombic Pb2(1)a-Na2FeSiO4/C, which is obtained from the conversion of C222(1)-Na-2 FeSiO4/C, is prepared and used as cathode material for the first time. Meanwhile, the pure phase C222(1)-Na2FeSiO4/C is firstly prepared by sol-gel method. Field emission scanning and transmission electron microscopy demonstrate that C222(1)-Na2FeSiO4/C has quasi-spherical morphology, and the active nanoparticles completely disperse within the cross-linked carbon nanolayer. The results from X-ray diffraction and Rietveld analysis reveal that the crystal structure of Pb2(1)a-Na2FeSiO4/C, similarly to C222(1)-Na2FeSiO4/C, can be formed by three-dimensional channel networks which generate from the alternate tetrahedral geometry chains of [FeO4] and (SiO4], but Pb2(1)a-Na2FeSiO4/C composite is more stable than C222(1)-Na2FeSiO4 /C. After the first sodiation/desodiation process, Pb2(1)a-Na2FeSiO4/C shows the specific capacity of 118 mAh g(-1) at 0.1 C and a capacity retention of 57.3% after 100 cycles.