Electrical and optical characteristics of vanadium in 4H-SiC

A semi-insulating layer is obtained in n-type 4H-SiC by vanadium-ion implantation. A little higher resistivity is obtained by increasing the annealing temperature from 1450 to 1650 ℃. The resistivity at room temperature is as high as 7.6 ×10^6 Ω. cm. Significant redistribution of vanadium is not observed even after 1650 ℃ annealing. Temperaturedependent resistivity and optical absorption of V-implanted samples are measured. The activation energy of vanadium acceptor level is observed to be at about Ec - 1.1 eV.

Theoretical study on the reaction of VS^+(~3∑^-,~1Γ)with CO

Two possible reaction mechanisms of VS^＋（^3∑^-, 1^Г） with CO in the gas phase have been studied by using B3LYP/TZVP and CCSD（T）/6-311＋G （3df, 3pd） methods： the O/S exchange reaction （VS^＋＋CO→VO^＋＋CS） and the S-transfer reaction （VS^＋ ＋ CO → V^＋ ＋ COS）. The two reactions proceed via two-step and one-step mechanism, respectively. The barriers of the triplet and singlet PESs are 30.6 and 50.9 kcal/mol, respectively, for O/S exchange reaction and 7.3 and 50.2 kcal/mol, respectively, for the S-transfer reaction. The results indicate that the triplet ground state reaction is more favorable, and the S-transfer reaction is more favorable than the O/S exchange reaction, which is in good agreement with the experimental observation.

Insights into the stable and fast lithium storage performance of oxygen-deficient LiV_(3)O_(8) nanosheets

Oxygen-deficient LiV_(3)O_(8) is considered as one of the promising cathode materials for lithium ion batteries(LIBs)because of its high cycling stability and rate capability.However,it is very difficult to control and study the content and position of V^(4+)and oxygen vacancies in LiV_(3)O_(8),and therefore the mechanism of improving electrochemical performance of LiV_(3)O_(8) is still unclear.Herein,we developed four LiV_(3)O_(8) nanosheets with different V^(4+)and oxygen vacancy contents and positions.The physicochemical and lithium storage properties indicate that the V^(4+)and oxygen vacancies in the surface layer increase the contribution of pseudocapacitive lithium storage on the nanosheet surface.The V^(4+)and oxygen vacancies in the lattice improve the electrical conductivity of LiV_(3)O_(8),and enhance the phase transformation and lithium ion diffusion rates.By adjusting the content of V^(4+)and oxygen vacancies,we obtained an oxygen-deficient LiV_(3)O_(8) nanosheet which maintained more than 93%of the initial reversible capacity after 300 cycles at 5,000 mA·g^(−1).The V^(4+)and oxygen vacancies play an important role in improving the stability and rapidity of lithium storage.This work is helpful to understand the stable and fast lithium storage mechanism of oxygen-deficient LiV_(3)O_(8),and might lay a foundation for further studies of other oxygen-deficient metal oxide electrodes for long-life and high-power LIBs.

Nanocomposite LiFePO_4·Li_3V_2(PO_4)_3/C synthesized by freeze-drying assisted sol-gel method and its magnetic and electrochemical properties

Nano-sized LiFePO_4·Li_3V_2（PO_4）_3/C was synthesized via a sol-gel route combining with freeze-drying. X-ray diffraction results show that this composite mainly consists of olivine Li Fe PO4 and monoclinic Li3 V2（PO4）3 phases with small amounts of V-doped LiFePO_4 and Fe-doped Li_3V_2（PO_4）_3. The magnetic properties of LiFePO_4·Li_3V_2（PO_4）_3/C are significantly different from LiFePO_4/C. Trace quantities of ferromagnetic impurities and Fe_2P are verified in LiFePO_4/C and LiFePO_4·Li_3V_2（PO_4）_3/C by magnetic tests, respectively. LiFePO_4·Li_3 V_2（PO_4）_3/C possesses relatively better rate capacities and cyclic stabilities, especially at high charge-discharge rates.The initial discharge capacities are 136.4 and 130.0 mA h g^（-1）,and the capacity retentions are more than 98% after 100 cycles at 2C and 5C, respectively, remarkably better than those of LiFePO_4/C. The excellent electrochemical performances are ascribed to the mutual doping of V^（3＋）and Fe^（2＋）, complementary advantages of LiFePO_4 and Li_3V_2（PO_4）_3 phases, the residual high-ordered carbon and Fe_2P with outstanding electric conductivity in the nanocomposite.