Haoyu Fu

MnO nanoparticles with cationic vacancies and discrepant crystallinity dispersed into porous carbon for Li-ion capacitors

MnO nanoparticles with cationic vacancies and discrepant crystallinity were prepared through a one-step hydrothermal synthesis followed by calcination at different temperatures. Glucose was used as both a reducing agent to introduce cationic vacancies with a content of ∼5.5% into MnO nanocrystals, and a carbon source to encapsulate MnO nanocrystals in a three dimensional porous framework. Cationic vacancies benefit phase transition in a conversion reaction, and together with a low degree of crystallinity, may also provide more void spaces for ion diffusion (3.37 × 10−13 cm2 s−1). Three dimensional porous carbon with a pore volume of 0.27 cm3 g−1 demonstrated a high electrical conductivity of 6.25 S cm−1 and offered fast pathways for charge transfer and penetration of the electrolyte. Such a synergistic structure endowed MnO with excellent electrochemical properties including a considerably enhanced capacity of 650 mA h g−1 at a current density of 1000 mA g−1. Li ion capacitors based on such a MnO anode and activated carbon cathode achieved the maximum energy density of 220 W h kg−1, and the capacitance retention was 95.3% after 3600 cycles at a rate of 5000 mA g−1.

Effects of Preinserted Na Ions on Li-Ion Electrochemical Intercalation Properties of V2O5

Na-preinserted V2O5 samples of NaxV2O5 (x = 0.00, 0.005, 0.01, or 0.02) were synthesized through sol-gel and freeze-drying routes and subsequent calcination. X-ray diffraction (XRD) results showed that all of the synthesized materials have typical orthorhombic structures without impurity phases. The lattice parameters were refined via the Rietveld refinement method, and the results suggested that the lattice parameters of preinserted samples increased in comparison with pristine V2O5. X-ray photoelectron spectroscopy (XPS) measurements demonstrated that the V(4+) concentration in the Na-preinserted V2O5 samples gradually increased as amount of sodium increased. Results from both XRD and XPS strongly suggested that Na ions indeed enter the interlamination position in the V2O5 crystal to expand the channels for Li-ion migration. NaxV2O5 samples exhibited improved electrochemical properties compared with those of pristine V2O5. Among all of the samples, Na0.01V2O5 delivered the highest reversible specific capacity, best cycling stability, and excellent rate capability. The analysis and discussion on ion diffusion revealed that the preinserted Na ions benefited the mobility of Li ions to improve the rate capabilities of electrodes.

Continuous Size Tuning of Monodispersed ZnO Nanoparticles and Its Size Effect on the Performance of Perovskite Solar Cells

ZnO has been demonstrated to be a promising candidate to fabricate high efficiency perovskite solar cells (PSCs) in terms of its better electron extraction and transport properties. However, the inability of synthesis of ZnO nanoparticles (NPs) with minimal surface defects and agglomeration remains a great challenge hindering the fabrication of highly efficient PSCs. In this work, highly crystalline and agglomeration-free ZnO NPs with controlled size were synthesized through a facile solvothermal method. Such ZnO NPs were applied in the fabrication of meso-structured PSCs. The solar cells with ∼40 nm ZnO NPs exhibit the highest power conversion efficiency (PCE) of 15.92%. Steady-state and time-resolved photoluminescence measurements revealed the faster injection and lower charge recombination at the interface of ∼40 nm ZnO NPs and perovskite, resulting in significantly enhanced JSC and VOC.

Impacts of Surface Energy on Lithium Ion Intercalation Properties of V2O5

Oxygen vacancies have demonstrated to be one of the most effective ways to alter electrochemical performance of electrodes for lithium ion batteries, though there is little information how oxygen vacancies affect the electrochemical properties. Vanadium pentoxide (V2O5) cathode has been investigated to explore the relationship among oxygen vacancies, surface energy, and electrochemical properties. The hydrogen-treated V2O5 (H-V2O5) sample synthesized via thermal treatment under H2 atmosphere possesses a high surface energy (63 mJ m(-2)) as compared to that of pristine V2O5 (40 mJ m(-2)) and delivers a high reversible capacity of 273.4 mAh g(-1) at a current density of 50 mA g(-1), retaining 189.0 mAh g(-1) when the current density increases to 2 A g(-1). It also displays a capacity retention of 92% after 100 cycles at 150 mA g(-1). The presence of surface oxygen vacancies increases surface energy and possibly serves as a nucleation center to facilitate phase transition during lithium ion intercalation and deintercalation processes.

Enhanced storage of sodium ions in Prussian blue cathode material through nickel doping

Prussian blue (PB) is a promising and cost-effective material for sodium ion batteries (SIBs) because it possesses fast diffusion channels for migration of Na ions and features a two-electron redox reaction mechanism that offers a high theoretical capacity of 170 mA h g−1. However, it is difficult to attain the full discharge capacity of PB in SIBs using the low-spin Fe2+/Fe3+ redox couple. In the present study, we found that doping PB with Ni ions (1–10%) resulted in enhanced electrochemical storage capacity and facilitated fast diffusion of Na ions during discharge. Specifically, PB doped with 3% Ni ions showed a discharge capacity of 117 mA h g−1, within which ≈50 mA h g−1 was attributed to the low-spin Fe2+C6/Fe3+C6 redox couple. Even though we do not know how to attain the full storage capacity of PB, this research sheds light on how substituting transition metal ions affects the electrochemical performance of PB. A new perspective of the electrochemical mechanism is also proposed for further understanding and improvement of its electrochemical performance.

Enhanced electrochemical properties of Li3VO4 with controlled oxygen vacancies as Li-ion battery anode

Li3 VO4 , as a promising intercalation-type anode material for lithium-ion batteries, features a desired discharge potential (ca. 0.5-1.0 V vs. Li/Li+ ) and a good theoretical storage capacity (590 mAh g-1 with three Li+ inserted). However, the poor electrical conductivity of Li3 VO4 hinders its practical application. In the present work, various amounts of oxygen vacancies were introduced in Li3 VO4 through annealing in hydrogen to improve its conductivity. To elucidate the influence of oxygen vacancies on the electrochemical performances of Li3 VO4 , the surface energy of the resulting material was measured with an inverse gas chromatography method. It was found that Li3 VO4 annealed in pure hydrogen at 400 °C for 15 min exhibited a much higher surface energy (60.7 mJ m-2 ) than pristine Li3 VO4 (50.6 mJ m-2 ). The increased surface energy would lower the activation energy of phase transformation during the charge-discharge process, leading to improved electrochemical properties. As a result, the oxygen-deficient Li3 VO4 achieved a significantly improved specific capacity of 495 mAh g-1 at 0.1 Ag-1 (381 mAh g-1 for pristine Li3 VO4 ) and retains 165 mAh g-1 when the current density increases to 8 Ag-1 .