A potential 4.2 V cathode material LiVPO4F for lithium batteries was prepared by two-step reaction method based on a carbon-thermal reduction （CTR） process. Firstly, V2O5, NH4H2PO4 and acetylene black are reacted un...A potential 4.2 V cathode material LiVPO4F for lithium batteries was prepared by two-step reaction method based on a carbon-thermal reduction （CTR） process. Firstly, V2O5, NH4H2PO4 and acetylene black are reacted under an Ar atmosphere to yield VPO4. The transition-metal reduction is facilitated by the CTR based on C→CO transition. These CTR conditions favor stabilization of the vanadium as V^3＋ as well as leaving residual carbon, which is useful in the subsequent electrode processing. Secondly, VPO4 reacts with ElF to yield LiVPO4F product. The property of the LiVPO4F was investigated by X-ray diffractometry （XRD）, scanning electron microscopy （SEM） and electrochemical measurement. XRD studies show that LiVPO4F synthesized has triclinic structure（space group p I ）, isostructural with the naturally occurring mineral tavorite, EiFePO4-OH. SEM image exhibits that the particle size is about 2μm together with homogenous distribution. Electrochemical test shows that the initial discharge capacity of LiVPO4F powder is 119 mA·h/g at the rate of 0.2C with an average discharge voltage of 4.2V （vs Ei/Li^＋）, and the capacity retains 89 mA·h/g after 30 cycles.展开更多
LiMn 2O 4-x F x prepared by the sol gel method has a perfect crystal formation .The crystal particle size of the material was medium and distributed uniformly. The substitution of F for O increased the specific...LiMn 2O 4-x F x prepared by the sol gel method has a perfect crystal formation .The crystal particle size of the material was medium and distributed uniformly. The substitution of F for O increased the specific capacity of the material at the cost of the cycleability .The explanation of this results is that the F decreases the valence of Mn,that is,more Mn 3+ and less Mn 4+ exist in the material.The increase of Mn 3+ will improve the initial specific capacity and Mn 3+ is the original reason for Jahn Teller effect that caused the poor cycleability of the cathode material by the micro distortion of the crystal structure. In addition, the expanded measurement of the crystal lattice is also the reason for the poor cycleability.Therefore,the results of F substitution and cation substitution are opposite.If the two methods are combined,they can compensate the inability each other and the satisfactory results may be obtained.展开更多
Optical properties of Pr^3＋ doped in Y2SiO5, including absorption spectra, emission spectra, and fluorescence decay, have been investigated with a special attention in the UV region. Broad band （270-350nm） UV fluor...Optical properties of Pr^3＋ doped in Y2SiO5, including absorption spectra, emission spectra, and fluorescence decay, have been investigated with a special attention in the UV region. Broad band （270-350nm） UV fluorescence assigned to the transitions 4f5d → 4f^2 ^3Hj, ^3Fj is found. The spontaneous transition probabilities in the 4f^2 intraconfiguration are calculated by utilizing the 3udd-Ofelt theory, by which three phenomenological parameters （Ω2, Ω4, Ω6,） are obtained by fitting the absorption spectra. For the evaluation of transition probability of the interconfiguration 4f5d → 4f^2, a formula for electric dipole transition is employed. In comparison of the measured fluorescence lifetime with the calculated spontaneous radiative lifetime, the fluorescence quantum efficiency is deduced.展开更多
In this study,a pseudo-layered Na super-ionic conductor of Na3V2(PO4)2F3 (NVPF)/C cathode for sodium-ion batteries is prepared successfully using a facile polyol refluxing process without any impurity phases.The X-ray...In this study,a pseudo-layered Na super-ionic conductor of Na3V2(PO4)2F3 (NVPF)/C cathode for sodium-ion batteries is prepared successfully using a facile polyol refluxing process without any impurity phases.The X-ray diffraction and Rietveld refinement results confirm that NVPF possesses tetragonal NASICON-type lattice with a space group of P42/mnm.In this preparative method,polyol is utilized as a solvent as well as a carbon source.The presence of nanosized NVPF particles in the carbon network is confirmed by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM).The existence of carbon is analyzed by Raman scattering and elemental analysis.When applied as a Na-storage material in a potential window of 2.0-4.3 V,the electrode exhibits two flat voltage plateaus at 3.7 and 4.2 V with an electrochemically active V^3+/V^4+ redox couple.In addition,Na3V2(PO4)2F3/C composite achieved a retention capacity of ~ 88% even after 1,500 cycles at 15 C.Moreover,at high current densities of 30 and 50 C,Na3V2(PO4)2F3/C cathode retains the specific discharge capacities of 108.4 and 105.9 mAh·g-1,respectively,revealing the structural stability of the material prepared through a facile polyol refluxing method.展开更多
基金Project(50302016) supported by the National Natural Science Foundation of China
文摘A potential 4.2 V cathode material LiVPO4F for lithium batteries was prepared by two-step reaction method based on a carbon-thermal reduction （CTR） process. Firstly, V2O5, NH4H2PO4 and acetylene black are reacted under an Ar atmosphere to yield VPO4. The transition-metal reduction is facilitated by the CTR based on C→CO transition. These CTR conditions favor stabilization of the vanadium as V^3＋ as well as leaving residual carbon, which is useful in the subsequent electrode processing. Secondly, VPO4 reacts with ElF to yield LiVPO4F product. The property of the LiVPO4F was investigated by X-ray diffractometry （XRD）, scanning electron microscopy （SEM） and electrochemical measurement. XRD studies show that LiVPO4F synthesized has triclinic structure（space group p I ）, isostructural with the naturally occurring mineral tavorite, EiFePO4-OH. SEM image exhibits that the particle size is about 2μm together with homogenous distribution. Electrochemical test shows that the initial discharge capacity of LiVPO4F powder is 119 mA·h/g at the rate of 0.2C with an average discharge voltage of 4.2V （vs Ei/Li^＋）, and the capacity retains 89 mA·h/g after 30 cycles.
文摘LiMn 2O 4-x F x prepared by the sol gel method has a perfect crystal formation .The crystal particle size of the material was medium and distributed uniformly. The substitution of F for O increased the specific capacity of the material at the cost of the cycleability .The explanation of this results is that the F decreases the valence of Mn,that is,more Mn 3+ and less Mn 4+ exist in the material.The increase of Mn 3+ will improve the initial specific capacity and Mn 3+ is the original reason for Jahn Teller effect that caused the poor cycleability of the cathode material by the micro distortion of the crystal structure. In addition, the expanded measurement of the crystal lattice is also the reason for the poor cycleability.Therefore,the results of F substitution and cation substitution are opposite.If the two methods are combined,they can compensate the inability each other and the satisfactory results may be obtained.
基金Supported by the National Natural Science Foundation of China under Grant No 10074020.
文摘Optical properties of Pr^3＋ doped in Y2SiO5, including absorption spectra, emission spectra, and fluorescence decay, have been investigated with a special attention in the UV region. Broad band （270-350nm） UV fluorescence assigned to the transitions 4f5d → 4f^2 ^3Hj, ^3Fj is found. The spontaneous transition probabilities in the 4f^2 intraconfiguration are calculated by utilizing the 3udd-Ofelt theory, by which three phenomenological parameters （Ω2, Ω4, Ω6,） are obtained by fitting the absorption spectra. For the evaluation of transition probability of the interconfiguration 4f5d → 4f^2, a formula for electric dipole transition is employed. In comparison of the measured fluorescence lifetime with the calculated spontaneous radiative lifetime, the fluorescence quantum efficiency is deduced.
文摘In this study,a pseudo-layered Na super-ionic conductor of Na3V2(PO4)2F3 (NVPF)/C cathode for sodium-ion batteries is prepared successfully using a facile polyol refluxing process without any impurity phases.The X-ray diffraction and Rietveld refinement results confirm that NVPF possesses tetragonal NASICON-type lattice with a space group of P42/mnm.In this preparative method,polyol is utilized as a solvent as well as a carbon source.The presence of nanosized NVPF particles in the carbon network is confirmed by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM).The existence of carbon is analyzed by Raman scattering and elemental analysis.When applied as a Na-storage material in a potential window of 2.0-4.3 V,the electrode exhibits two flat voltage plateaus at 3.7 and 4.2 V with an electrochemically active V^3+/V^4+ redox couple.In addition,Na3V2(PO4)2F3/C composite achieved a retention capacity of ~ 88% even after 1,500 cycles at 15 C.Moreover,at high current densities of 30 and 50 C,Na3V2(PO4)2F3/C cathode retains the specific discharge capacities of 108.4 and 105.9 mAh·g-1,respectively,revealing the structural stability of the material prepared through a facile polyol refluxing method.
