Synthesis And Performance Of Na₃V₂（PO₄）₃/C Cathode Materials For Sodium-ion Batteries

Lithium-ion batteries (LIBs) have been used widely in portable electronic devices,owing to their excellent energy-storage performance. However， lithium is notavailable in the amounts necessary for meeting the increasing market demand. Thus,rechargeable batteries based on alternative, abundantly available materials need to bedeveloped. Sodium-ion batteries (NIBs) are a promising alternative to LIBs forlarge-scale energy storage applications, because of the high availability of sodium, itslow cost, and the similarity in the intercalation chemistry of sodium and lithium.Rhombohedral Na3V2(PO4)3(NVP), which has a NASICON-type frameworkstructure, has a robust structure, good ionic mobility, a high theoretical specificcapacity, and a high operating voltage. As a result, it should be a suitable cathodicmaterial for NIBs. In this study, the carbon coating technology, such as solid-phasesynthesis assisted by mechanochemical activation and combustion synthesis, is usedto improve the electrical conductivity of this material, thus resulting in excellentelectrochemical performance. The main research results obtained as follows:First, the solid-phase synthesis assisted by mechanochemical activation was used tosynthesize Na3V2(PO4)3/C (NVP/C) composite. The results showed that the samplewith15wt%glucose content, synthesized at900°C for4hours, exhibited betterelectrochemical performance. The NVP/C particles, whose average size was300nm,had an irregular morphology and uniform particle size distribution. These particleswere uniformly coated with a layer of amorphous carbon and this coating layer wasapproximately3nm in thickness. This carbon shell layer not only restricts the growthof the NVP crystalline particles and prevents them from agglomerating during theannealing process, but also enhances the electronic contact between the particles,hence improve electrochemical performance of this material. The NVP/C compositeexhibited initial charge capacities of115.18mAh g1for potentials ranging from2.5to3.8V, which is very close to the theoretical capacity (117.6mAh g1) of NVP. Italso exhibited initial discharge capacities of98.12mAh g1and only4.92%of itsinitial discharge capacity was lost during50thcycle. The initial discharge capacitiesfor the sample at the current rates of10,20, and30C were88.21,84.90, and76.44mAh g1, respectively；The initial discharge capacities at these rates were muchhigher than those reported in the literature. The NVP/C composite exhibited highspecific capacity and excellent rate cycling performance. Second, the NVP/C cathodic material has been successfully synthesized for use insodium-ion batteries (NIBs) via a glucose-assisted combustion process using low-costcitrate acid as fuels. The sample with25wt%glucose content, synthesized at900°Cfor4hours, exhibited better energy-storge performance. These NVP/C particles withuniform particle size distribution, whose average size was around400nm, wereuniformly coated with an amorphous carbon shell layer. This residual carbon shelllayer (ca.3nm in thickness), generated in situ from the glucose, not only retarded thegrowth of NVP particles, but also provided good electronic contact between materialparticles, leading to a high electronic conductivity. The well carbon-coating NVPsamples delivered initial charge capacity of111.30mAh g1and an attractive initialdischarge capacity of100.72mA h g-1for potentials ranging from2.5to3.8V at0.1Crate, and approximately94.9%of its initial specific capacity was retained after50thcycles. The initial discharge capacities for the samples were84.36,74.49and61.76mAh g1at10,20and30C rate, respectively, indicating its excellent rate and cyclingperformance.Finally, the electrode made from this NASICON-type material displayed two pairsof well-defined redox peaks between1.2and3.8V. The oxidation (Na extraction) andreduction (Na insertion) peaks located at around3.4V (vs. Na+/Na) can be assigned tothe V4+/V3+redox couple on the basis of the two-phase transition betweenNa3V2(PO4)3and NaV2(PO4)3. And the redox peaks located at approximately1.6Vcould be attributed to the V3+/V2+redox couple, which corresponds to thecompositional transition between Na3V2(PO4)3and Na4V2(PO4)3. It is known thatwell-defined oxidation and reduction peaks as well as small differences in thepolarisation voltages corresponding to the peaks, which depend on the transitionsbetween the V3+/V4+and V2+/V3+redox states, are indicative of outstandingreversibility with respect to the carbon-assisted extract/insertion of Na ions. Thecalculated DNa(diffusion coefficient of sodium ions) values obtained using EIS andCV measurements fall within the order of1010–1012cm2s1, which were3to4orders of magnitude larger than other electrode materials for sodium ion batteries(such as NaFePO4,8.63×10-17cm2s–1), demonstrating its higher diffusion coefficientof sodium ions. Simulation results showd that NASICON-type Na3V2(PO4)3had openthree-dimensional (3D) framework structure. This structure provides3D well-definedion channels, which act as pathways for the diffusion of Na ions. The3D frameworkstructure of NVP is not damaged during Na+extraction/insertion processes. The volumetric change during the two-phase transition between Na3V2(PO4)3andNa1V2(PO4)3is9.65%. And volumetric change during the compositional transitionbetween Na3V2(PO4)3and Na4V2(PO4)3is2.72%. This suggests that the smallvolumetric change might facilitate the diffusion of Na ions in the structure and causeminimal distortion of the crystal lattice. Thus, using the synthesised NVP/C powderswith a stable NASICON-type structure as an electrode material for NIBs should resultin superior charge-discharge cycling performance.