Enhanced electrode kinetics and properties via anionic regulation in polyanionic Na_(3+x)V_(2)(PO_(4))_(3-x)(P_(2)O_(7))_(x) cathode material

Mixing polyanion cathode materials are promising candidates for the development of next-generation batteries, owing to their structural robustness and low-volume changes, yet low conductivity of polyanion hinders their practical capacity. Herein, the anion-site regulation is proposed to elevate the electrode kinetics and properties of polyanionic cathode. Multivalent anion P_(2)O_(7)^(4-) is selected to substitute the PO_(4)^(3-) in Na_(3)V_(2)(PO_(4))_(3) (NVP) lattice and regulate the ratio of polyanion groups to prepare Na_(3+x)V_(2)(PO_(4))_(3-x)(P_(2)O_(7))_(x)(NVPP_(x), 0 ≤ x ≤ 0.15) materials.The optimal Na_(3.1)V_(2)(PO_(4))_(2.9)(P_(2)O_(7))_(0.1) (NVPP_(0.1)) material can deliver remarkably elevated specific capacity(104 mAh g^(-1) at 0.1 C, 60 mAh g^(-1) at 20 C, respectively), which is higher than those of NVP. Moreover, NVPP_(0.1) exhibits outstanding cyclic stability(91% capacity retention after 300 cycles at 1 C). Experimental analyses reveal that the regulation of anions improves the structure stability, increases the active Na occupancy in the lattice and accelerates the Na+migration kinetics. The strategy of anion-site regulation provides the researchers a reference for the design of new high-performance polyanionic materials.

Sulfur-modified nickel selenide as an efficient electrocatalyst for the oxygen evolution reaction

The sluggish four-electron transfer of the oxygen evolution reaction(OER)limits the performance of water electrolyzers.Hence,OER electrocatalysts based on earth-abundant elements are urgently needed.Heteroatom doping has been an efficient approach to boost the intrinsic OER activity of the active sites by modifying the electronic structure.Here,a simple anion substitution strategy is reported that increases the OER activity of nickel selenides via a one-step hydrothermal treatment of a metal–organic framework precursor.The resulting S-substituted Ni_(3)Se_(4) nanoparticles display distortion of their crystal lattice.As expected,the sulfur substitution modifies the electronic structure of Ni_(3)Se_(4) and leads to outstanding electrocatalytic activity.All the S-substituted Ni_(3)Se_(4) catalysts exhibit higher OER activities than the original Ni_(3)Se_(4).The optimized catalyst achieves a current density of 10 mA cm^(−2) at an overpotential of 275 mV with a Tafel slope of 64 mV dec^(−1) in 1.0 M KOH.In addition to its electrochemical activity,the S-Ni_(3)Se_(4)-2 catalyst also exhibits good stability with only a 7.5%increase in overpotential at 50 mA cm^(−2) after 100 hours.This work demonstrates one strategy to modify the electronic structure of transition metal compounds by anion regulation.

Effect of anionic group[SiO_(4)]^(4-)/[PO_(4)]^(3-) on the luminescence properties of Dy^(3+)-doped tungstate structural compounds

Novel scheelite structures of Li_(2)Ca(WO_(4))_(2),Li_(2)Ca_(2)(WO_(4))(SiO_(4)),and LiCa_(2)(WO_(4))(PO_(4))fluorescent materials were successfully prepared using a high-temperature solid-phase process.The compounds were characterized by X-ray diffraction and energy dispersive spectroscopy.The tests revealed that the substitution of[WO_(4)]^(2-)by[SiO_(4)]^(4-) or [PO_(4)]^(3-) tetrahedron in tungstate had no significant influence on the crystal structure of the Li_(2)Ca(WO_(4))_(2).When Dy^(3+) ions were introduced as an activator at an optimum doping concentration of 0.08 mol%,all of the as-prepared phosphors generated yellow light emissions,and the emission peak was located close to 576 nm.Replacing[WO_(4)]^(2-) with [SiO_(4)]^(4-) or [PO_(4)]^(3-) tetrahedron significantly increased the luminescence of the Li_(2)Ca(WO_(4))_(2) phosphors.Among them,the LiCa_(2)(WO_(4))(PO_(4)):0.08Dy^(3+) phosphor had the best luminescence properties,decay life(r=0.049 ms),and thermal stability(87.8%).In addition,the as-prepared yellow Li_(2)Ca(WO_(4))_(2):0.08Dy^(3+),Li_(2)Ca_(2)(WO_(4))(SiO_(4)):0.08Dy^(3+),and LiCa_(2)(WO_(4))(PO_(4)):0.08Dy^(3+) phosphor can be used to fabricate white light emitting diode(LED)devices.

Enhanced thermoelectric properties of Zintl phase YbMg_(2)Bi1.98 through Bi site substitution with Sb

Benefiting from the unique"Phonon-Glass,Electron-Crystal"(PGEC)characteristic,Zintl phases have been considered as a kind of promising thermoelectric materials.For the typical AM2X2 compounds with the CaAl2Si2-type structure,YbMg_(2)Bi2 has shown competitive thermoelectric performance recently.Nevertheless,the optimization of YbMg_(2)Bi2 compounds is primarily focused on the substitution on Yb or Mg site.Herein,the Bi site is substituted by isoelectric Sb and the effect on the thermoelectric transport behavior is investigated.The partial substitution reduces the carrier concentration and induces the lattice deformation caused by the different atomic radius and mass between Bi and Sb,further leading to the decreased power factor and thermal conductivity.Fortunately,the reduction extent of the thermal conductivity outperforms that of power factor.Finally,the Sb substitution successfully results in a better thermoelectric performance compared with that of the pristine YbMg_(2)Bi1.98.Especially,the calculated energy conversion efficiency(a)of YbMg_(2)Bi1.88Sb0.1 which also possesses a relatively high output power density reaches the maximum value of 9.8% when Th=873 K,and Tc=300 K,respectively.This work demonstrates that the idea of substitution on anionic site should be a new strategy to achieve better ZT values for AM2X2 compounds.