| dc.description.abstract | ABSTRACT
The energy crisis and high levels of pollution resulting from fossil fuels have boosted the government to develop renewable energies (i.e., solar tide and wind). The lithium-ion battery is an energy storage system that has been widely applied for mobile phone, laptop computer, and electric vehicles. However, lithium resources are expensive and geographically constrained, making the application of lithium-ion in massive scale electrical energy storage systems (EES) needs to be reconsidered. On the other hand, sodium batteries are currently getting much attention due to global abundance and low cost. Moreover, the working principle of sodium-ion batteries and lithium-ion batteries are alike, making them promising to be developed as an energy storage system.
Na3V2(PO4)2F3/C with a 3D NASICON (Na-superionic conductor) type framework provides sufficient space to accommodate Na+ ions, making the material potentially a cathode material. Na3V2(PO4)2F3/C also has high energy density and excellent cycling performance. However, capacity reduction and inferior rate capability due to low electronic conductivity make this material difficult to implement practically. Here, sodium storage delivered excellent performance was realized by substituting the Mg2+ ion into the Na3V2(PO4)2F3/C structure, synthesized by a combination of sol-gel and carbon-thermal reduction. We studied the role of magnesium ions in the Na3V2(PO4)2F3/C structure and electrochemical properties in detail. The Mg2+ ion, which is substituted at the V site, increases the electronic conductivity and ionic conductivity with resulting hole and broadening the ionic diffusion pathway for Na+ ions in the lattice crystal. An excellent rate capability of 80 mAh g-1 at 10 C was achieved by Na3V1.95Mg0.05(PO4)2F3/C electrode; In addition, the retention capacity still reaches 88% after 500 cycles and the average coulombic efficiency is 99.9%. This achievement is related to multiple effect of Mg-doped Na3V2(PO4)2F3/C : (1) elevating the bulk electronic conductivity; (2) boosting the Na+ ion diffusion in the crystal structure; (3) reducing the crystal size and particle size: (3) improving the structural stability.
Besides the Mg2+ as a dopant atom, the electrochemical enhancement of Na3V2(PO4)2F3 was conducted by Ca2+ doped Na3V2(PO4)2F3. Despite the large size of the Ca2+ ion, the XRD pattern showed no change in the crystal structure. Moreover, the Ca2+ doped NVPF (x = 0.05) electrode delivered the highest capacities 124 mAh g-1 and 86 mAh g-1 at 0.1 C and 10 C, respectively, due to the enlargement of the crystal lattice and smaller particle size. | en_US |