Lanli Chen

Boron Substituted Na3V2(P1−xBxO4)3 Cathode Materials with Enhanced Performance for Sodium-Ion Batteries

The development of excellent performance of Na‐ion batteries remains great challenge owing to the poor stability and sluggish kinetics of cathode materials. Herein, B substituted Na3V2P3 –xBxO12 (0 ≤ x ≤ 1) as stable cathode materials for Na‐ion battery is presented. A combined experimental and theoretical investigations on Na3V2P3 –xBxO12 (0 ≤ x ≤ 1) are undertaken to reveal the evolution of crystal and electronic structures and Na storage properties associated with various concentration of B. X‐ray diffraction results indicate that the crystal structure of Na3V2P3 –xBxO12 (0 ≤ x ≤ 1/3) consisted of rhombohedral Na3V2(PO4)3 with tiny shrinkage of crystal lattice. X‐ray absorption spectra and the calculated crystal structures all suggest that the detailed local structural distortion of substituted materials originates from the slight reduction of V–O distances. Na3V2P3‐1/6B1/6O12 significantly enhances the structural stability and electrochemical performance, giving remarkable enhanced capacity of 100 and 70 mAh g−1 when the C‐rate increases to 5 C and 10 C. Spin‐polarized density functional theory (DFT) calculation reveals that, as compared with the pristine Na3V2(PO4)3, the superior electrochemical performance of the substituted materials can be attributed to the emergence of new boundary states near the band gap, lower Na+ diffusion energy barriers, and higher structure stability.

Hydrogen-doping induced reduction in the phase transition temperature of VO2: a first-principles study

VO2 is a promising thermochromic material that can intelligently control the transmittance of sunlight in the near-infrared region in response to temperature change, although the high phase transition temperature (Tc) of 340 K restricts its wide application. Our first-principles calculations show that hydrogen is an efficient dopant which can stabilize the metallic VO2 phase at ambient temperature through reducing Tc by 38 K/at% H. The reduction in Tc is coupled with the changes in atomic and electronic structures, i.e., the V-V chains feature the dimerization characteristics in H-doped VO2(R) and the V-O bonds become less ionic due to the formation of a typical H-O covalent bond. In addition, hydrogen-doped VO2 is more sensitive to external strain as compared with pure VO2, implying that Tc can be further regulated through a combination of H-doping and strain.

Tuning the phase transition temperature, electrical and optical properties of VO2 by oxygen nonstoichiometry: insights from first-principles calculations

Vanadium dioxide (VO2) is one of the most promising thermochromic materials with a reversible metal–insulator transition (MIT) from a high-temperature rutile phase to a low-temperature monoclinic phase, although a high MIT temperature (Tc) of 340 K for bulk VO2 restricts its wide application. Our first-principles calculations show that the oxygen nonstoichiometry plays an important role in tuning the MIT behavior of VO2. The O-vacancy in bulk VO2 gives rise to an increase in electron concentration, which induces a decrease in Tc. On the other hand, O-vacancy and O-adsorption on VO2(R) (1 1 0) and VO2(M) (0 1 1) surfaces could alter their work functions and in turn regulate Tc. In addition, the formation and adsorption energies of O-adsorption on the two types of surfaces are negative, indicating that VO2 surfaces are prone to oxidation in ambient air. The present results contribute to both tuning phase transition behaviors experimentally and reducing hindrances for the advanced applications of VO2-based materials.

Low temperature fabrication of thermochromic VO2 thin films by low-pressure chemical vapor deposition

VO2(M) is of special interest as the material with the most potential for future application in smart windows and switching devices. However, a number of drawbacks need to be overcome, including the high processing temperature of current synthesis techniques and low thermochromic properties. This work reports the fabrication of high-performance thermochromic VO2 thin films at low temperatures below 400 °C based on a low-pressure chemical vapor deposition (LPCVD) with a vanadium(III) acetylacetonate precursor. Proper tuning of the process parameters is found to be critical in fabricating thickness-controllable highly-crystalline VO2 films. For an ∼62 nm thick VO2 film, visible transmittances of 52.3% (annealed at 400 °C) and 52.7% (annealed at 350 °C) were obtained. The corresponding solar energy modification abilities (ΔTsol) were 9.7% and 7.1%, and the transition temperatures were 45.1 °C and 50.9 °C. The underlying microscopic mechanism was studied by first-principles calculations and the results indicated that improved performances, including a low transition temperature, could be achieved by properly controlling the annealing temperature, ascribed to the combined effect of strain and oxygen vacancies. Moreover, the initial use of a pre-grown seed layer induced fast grain growth, which is favorable for further decreasing the deposition and annealing temperature to 325 °C.

An α-CrPO4-type NaV3(PO4)3 anode for sodium-ion batteries with excellent cycling stability and the exploration of sodium storage behavior

A novel α-CrPO4-type vanadium-based orthophosphate NaV3(PO4)3 was synthesized as a promising anode material for Na-ion batteries, which exhibits a high reversible capacity of 140 mA h g−1 with the capacity retention about 98% after 100 cycles. Furthermore, the electrochemical reaction mechanism, structure evolution and ionic diffusion of NaV3(PO4)3 during the sodiation process were surveyed by the combination of experiments and first-principles calculations. Two different Wyckoff sites (4b, Na2 and 4c, Na3) which can accommodate more sodium ions, as well as the corresponding Na insertion process were reported to understand the sodium storage mechanism in NaV3(PO4)3. The 3d-electron distribution near the Fermi level for vanadium in different valence states and sites (4a and 8g) in NaxV3(PO4)3 (x = 1, 2, 3) and the local electron transfer during the sodiation process were described to predict the transition of the electrochemical properties. The cooperative-transport mechanism dominates the Na diffusion in NaxV3(PO4)3 (1 ≤ x ≤ 3) and the activation barrier for Na+ transport in Na1.125V3(PO4)3 (0.30 eV) is much lower than that in NaV3(PO4)3 (1.28 eV), suggesting the fast-ion transport characteristics during the sodiation process and the strong stability of Na at the 4e site.

First-principles study of the effect of oxygen vacancy and strain on the phase transition temperature of VO2

Vanadium dioxide (VO2) has been extensively studied as a thermochromic material due to its metal–insulator transition at a critical temperature (Tc) of ∼340 K. Our first-principles calculations show that V–V chains in rutile VO2 with oxygen vacancy (VO2−x(R)) exhibit dimerization, and the band gap of monoclinic VO2 with oxygen vacancy (VO2−x(M)) can be narrowed to 0.51 eV compared to the 0.69 eV of pure monoclinic VO2 (VO2(M)), resulting in increased near-IR absorption. Furthermore, the smaller energy barrier of oxygen vacancy in VO2−x(M) (0.51 eV) with respect to VO2−x(R) (0.55 eV) indicates that VO2−x(M) could easily capture the oxygen from air and transition back to the normal VO2(M). However, precisely the opposite case is found for VO2−x(R), such that an oxygen vacancy in VO2−x(R) can stabilize the rutile phase at a low temperature. In addition, VO2−x is more sensitive to strain than pure VO2, implying that combining the effect of the oxygen vacancy and compressive strain could effectively tune the phase transition behavior and further reduce its phase transition temperatures. These results provide a theoretical guidance for the improvement of smart devices.

Formation energies of intrinsic point defects in monoclinic VO2 studied by first-principles calculations

VO2 is an attractive candidate for intelligent windows and thermal sensors. There are challenges for developing VO2-based devices, since the properties of monoclinic VO2 are very sensitive to its intrinsic point defects. In this work, the formation energies of the intrinsic point defects in monoclinic VO2 were studied through the first-principles calculations. Vacancies, interstitials, as well as antisites at various charge states were taken into consideration, and the finite-size supercell correction scheme was adopted as the charge correction scheme. Our calculation results show that the oxygen interstitial and oxygen vacancy are the most abundant intrinsic defects in the oxygen rich and oxygen deficient condition, respectively, indicating a consistency with the experimental results. The calculation results suggest that the oxygen interstitial or oxygen vacancy is correlated with the charge localization, which can introduce holes or electrons as free carriers and subsequently narrow the band gap of mono...