A dual-modification strategy for P2-type layered oxide via bulk Mg/Ti co-substitution and MgO surface coating for sodium ion batteries.

P2-type materials are regarded as competitive cathodes for next generation sodium ion batteries. However, the unfavorable P2 â†’ O2 phase transition usually leads to severe capacity decay. Moreover, the cathode material always suffers from destruction of surface crystal structure caused by trace amount of HF. In this study, a dual-modification method containing Mg/Ti co-doping and MgO surface coating is designed to solve the defects of P2-type Na0.67Ni0.17Co0.17Mn0.66O2 cathode. Results turn out that the P2 structure can be stabilized via Mg/Ti co-substitution and MgO layer could effectively prevent the surface from corroding by HF and promote migration of Na+. Moreover, the as-prepared MgO-coated Na0.67Ni0.17Co0.17Mn0.66Mg0.1O2 exhibits improved electrochemical performance than the raw material. It delivers 111.6 mAh g-1 initial discharge capacity and maintains 90.6% at high current density of 100 mA g-1 within 2-4.5 V, which has been obviously enhanced than that of Na0.67Ni0.17Co0.17Mn0.66O2. The significant improvement can be attributed to the synergistic effect of Mg/Ti co-substitution and MgO surface coating. This dual-modification strategy based on the synergetic effect of Mg/Ti co-doping and MgO surface coating might be a resultful step forward to develop cathode materials for sodium ion batteries.

Facile Synthesis of Nitrogen-Doped Green-Emission Carbon Dots as Fluorescent Off-On Probes for the Highly Selective Sensing Mercury and Iodine Ions.

In this work, green-emission carbon dots (CDs) were prepared for detecting mercury ions (Hg(II)) and iodine ions via a facile hydrothermal method using ethylenediamine and methyl red as nitrogen and carbon sources, respectively, without any other complex reagents. The bacteriostasis experiment showed that the CDs were not toxic to the growth of four kinds of bacteria: Staphylococcus aureus (S. aureus), Bacillus subtilis (B. subtilis), Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa), which indicated that the CDs had good security and could be used for analysis and detection. The CDs exhibited excitation-independent emission, the fluorescence of which could be quenched by Hg(II), and could be recovered by iodine ions. An approach was established to detect Hg(II) based on the fluorescence quenching of CDs by the synergistic action of a photo induced electron transfer (PET) mechanism, and iodine ions were detected based on the fluorescence recovery of CDs by a HgI2 precipitate formation mechanism. The detection limits for Hg(II) and iodine reached 0.89 µM and 0.50 µM, respectively. Compared to most methods, the method mentioned in this paper has good selectivity, a wider linear range, a lower detection limit and higher security. The synthesized CDs could be probes for sensing Hg(II) and iodine ions.