Xiaobiao Wu

Sol–gel synthesis and electrochemical properties of fluorophosphates Na2Fe1−xMnxPO4F/C (x = 0, 0.1, 0.3, 0.7, 1) composite as cathode materials for lithium ion battery

Fluorophosphates Na2Fe1−xMnxPO4F/C (x = 0, 0.1, 0.3, 0.7, 1) composite were successfully synthesized via a sol–gel method. The structure, morphology and electrochemical performance of the as prepared materials were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and charge/discharge measurements. XRD results show that, consistent with Na2FePO4F, Na2Fe0.9Mn0.1PO4F (x = 0.1) crystallize in a two-dimensional (2D) layered structure with space groupPbcn. However, increasing the content of Mn to x ≥ 0.3 results in a structure transition of Na2Fe1−xMnxPO4F from the 2D layered structure of Na2FePO4F to the three-dimensional (3D) tunnel structure of Na2MnPO4F. SEM and TEM analysis indicates nanostructured primary particles (about tens of nanometres in diameter) are obtained for all samples due to uniform carbon distribution and low calcining temperature used. Na2FePO4F is able to deliver a reversible capacity of up to 182 mA h g−1 (about 1.46 electrons exchanged per unit formula) with good cycling stability. Compared with Na2FePO4F, partial replacement of Fe by Mn in Na2Fe1−xMnxPO4F increases the discharge voltage plateau. Similar to Na2FePO4F, iron-manganese mixed solid solution Na2Fe1−xMnxPO4F (x = 0.1, 0.3, 0.7) also show good cycling performance. Furthermore, Na2MnPO4F with high electrochemical activity was successfully prepared for the first time, which is able to deliver a discharge capacity of 98 mA h g−1. The good electrochemical performance of Na2Fe1−xMnxPO4F materials can be attributed to the distinctive improvement of ionic/electronic conduction of the materials by formation of nanostructure composite with carbon.

Exploiting Na2MnPO4F as a high-capacity and well-reversible cathode material for Na-ion batteries

A Na2MnPO4F/C nanocomposite material is successfully synthesized via spray drying, followed by a high-temperature sintering method. It is shown that the highly phase-pure Na2MnPO4F with symmetry of the P21/n space group is uniformly embedded in the carbon networks, which play a key role in building up a highly efficient, electron-flow channel and elevating the electronic conductivity of the nanocomposites. The electrochemical measurements show that the initial discharge capacity of Na2MnPO4F reaches up to 140 and 178 mA h g−1 at 30 °C and 55 °C, respectively. Furthermore, the capacity still maintains 135 mA h g−1 after 20 cycles at 55 °C. The Na+ diffusion coefficient in Na2MnPO4F is calculated at about 10−17 cm2 s−1 by the GITT method. The impressive cycling performance of the material is ascribed to the good structural reversibility and stability of Na2MnPO4F, which are confirmed by the ex situ XRD measurements during the first cycle and after 30 cycles.

Promoting long-term cycling performance of high-voltage Li2CoPO4F by the stabilization of electrode/electrolyte interface

High-voltage Li2CoPO4F (∼5 V vs. Li/Li+) with double-layer surface coating has been successfully prepared for the first time. The Li3PO4-coated Li2CoPO4F shows a high reversible capacity of 154 mA h g−1 (energy density up to 700 W h kg−1) at 1 C current rate, and excellent rate capability (141 mA h g−1 at 20 C). XRD and MAS NMR results show that Li2CoPO4F can be indexed as an orthorhombic structure with space group Pnma and coexists with Li3PO4. The XPS depth profiles and TEM analysis reveal that the as-prepared material has a double-layer surface coating, with a carbon outer layer and a Li3PO4 inner layer, which greatly enhances the transfer kinetics of the lithium ions and electrons in the material and stabilizes the electrode/electrolyte interface. Using LiBOB as an electrolyte additive is another way to further stabilize the electrode/electrolyte interface, and the LiBOB has a synergistic effect with the Li3PO4 coating layer. In this way, the Li2CoPO4F cathode material exhibits excellent long-term cycling stability, with 83.8% capacity retention after 150 cycles. The excellent cycling performance is attributed to the LiBOB electrolyte additive and the Li3PO4 coating layer, both of which play an important role in stabilizing the charge transfer resistance of Li2CoPO4F upon cycling.

Synthesis, orientation and conductivity investigation of a new porphyrin Langmuir-Blodgett film

A new amphiphilic porphyrin with only one hydrophilic substituent is synthesized and the orientation and conductivity of its Langmuir-Blodgett film are investigated. The results show that the porphyrin rings stand on the surface with an orientation angle of less than 16° with respect to the surface normal, and adopt a face-to-face stacking arrangement in the direction of compression. The film has a high degree of conductivity anisotropy. After doping with iodine vapour, the in-plane conductivity parallel to the compression direction is 3.95 × 10–4 S cm–1, and that perpendicular to it is 1.34 × 10–8 S cm–1.

Novel Phosphamide Additive to Improve Thermal Stability of Solid Electrolyte Interphase on Graphite Anode in Lithium-Ion Batteries

In this communication, a novel electrolyte additive, N,N-diallyic-diethyoxyl phosphamide (DADEPA), is described for the first time to improve the thermal stability of lithiated graphite anode in Li-ion batteries. The differential scanning calorimetry (DSC) measurement demonstrated that when the graphite was lithiated in the 5% DADEPA-containing electrolyte, the heat generation decreased sharply by half as compared with the reference, whereas the onset temperature for the main exothermic process was postponed by 20 °C. Electrochemical and XPS analyses indicated that the distinctive improvement in thermal safety came from a new interfacial chemistry, in which phosphorus-containing ingredients was embedded during the initial forming of the interphase.