Chuying Ouyang

Ab initio studies on atomic and electronic structures of black phosphorus

The atomic and electronic properties of black phosphorus (BP), which has been recently shown to have potential application as anode material for lithium ion batteries, are studied via ab initio calculations. The calculations reveal that the interlayer interaction in BP is Van der Waals Keesom force, which is critical to the formation of the layered structure. Interestingly, we also found that the small band gap of bulk BP (0.19 eV) when compared with that of single layer BP (0.75 eV) is partly because of the interlayer Van der Waals interaction in BP. The change in a materials band structure because of Van der Waals interaction is rarely reported in literature.

Effect of Co Content on Rate Performance of LiMn0.5 − x Co2x Ni0.5 − x O 2 Cathode Materials for Lithium-Ion Batteries

Layer-structured LiMn0.5-xCo2xNi0.5-xO2 was prepared as cathode material for lithium-ion batteries. The structures of the layered materials and the oxidation states of the elements in the compounds were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. Adsorbed oxygen was detected on the surface of material. With the increase of Co content in LiMn0.5-xCo2xNi0.5-xO2, the oxidation state of Ni, Mn, Co, and O became higher gradually while the amount of oxygen adsorbed on the surface of LiMn0.5-xCo2xNi0.5-xO2 grains reduced obviously. Electrochemical evaluation showed that addition of Co in LiMn0.5-xCo2xNi0.5-xO2 is beneficial to its rate performance. The variations of the electronic structure of Ni, Mn, and O may be responsible for the improvement of the rate capability in LiMn0.5-xCo2xNi0.5-xO2 with addition of Co

Ab initio studies on Li4+xTi5O12 compounds as anode materials for lithium-ion batteries.

The structural and electronic properties of Li(4+) (x)Ti(5)O(12) compounds (with 0<or=x<or=6)-to be used as anode materials for lithium-ion batteries-are studied by means of first principles calculations. The results suggest that Li(4)Ti(5)O(12) can be lithiated to the state Li(8.5)Ti(5)O(12), which provides a theoretical capacity that is about 1.5 times higher than that of the compound lithiated to Li(7)Ti(5)O(12). Further insertion of lithium species into the Li(8.5)Ti(5)O(12) lattice results in a clear structural distortion. The small lattice expansion observed upon lithium insertion (about 0.4 % for the lithiated material Li(8.5)Ti(5)O(12)) and the retained [Li(1)Ti(5)](16d)O(12) framework indicate that the insertion/extraction process is reversible. Furthermore, the predicted intercalation potentials are 1.48 and 0.05 V (vs Li/Li(+)) for the Li(4)Ti(5)O(12)/Li(7)Ti(5)O(12) and Li(7)Ti(5)O(12)/Li(8.5)Ti(5)O(12) composition ranges, respectively. Electronic-structure analysis shows that the lithiated states Li(4+x)Ti(5)O(12) are metallic, which is indicative of good electronic-conduction properties.

First-principles study of the oxygen adsorption and dissociation on graphene and nitrogen doped graphene for Li-air batteries

Recently, Yoo and Zhou [ACS Nano 5, 3020–3026 (2011)] reported that graphene can be directly used as electrode for Li-air batteries, suggesting that graphene itself can be served as catalyst for O2 dissociation reaction. In this work, we show from density functional theory calculations that the O2 dissociation reaction energy barrier is substantially decreased on the graphene surface comparing to that of in vacuum. Furthermore, N-doping can further decrease the energy barrier from 2.39 eV (undoped case) to 1.20 eV. The O2 molecule physical adsorption, O atom chemical adsorption at the graphene, and N-doped graphene surfaces are also simulated.

The effect of Cr doping on Li ion diffusion in LiFePO4 from first principles investigations and Monte Carlo simulations

Using the adiabatic trajectory method, the migration energy barriers for the migration of Li ions and Cr ions along the one-dimensional diffusion pathway in pure and Cr doped LiFePO4 are obtained from first principles calculations. The results show that while Li ions can diffuse along the diffusion pathway easily, Cr ions do not easily diffuse away from their initial positions. This means that the heavy Cr ions will block the one-dimensional diffusion pathway of the material. Monte Carlo simulations are performed to evaluate the influences of the blocking behaviours on the electrochemical performance of LiFePO4 cathode material for Li ion secondary batteries. The results show that the evaluated capacity is highly sensitive to the amount of the dopant, the size of the super-cell being used for simulation (particle size of the powder cathode material) and the Monte Carlo steps for statistics (charge–discharge current density).

Strain-induced semimetal-metal transition in silicene

The effect of the tensile strain on the electronic structure of the silicene is studied by using first-principles density functional theory. It is found that a semimetal-metal transition occurs when an in-plane strain larger than 7.5% is applied in silicene. The downward movement of the lowest conduction band at Γ-point, which originates from the weakened interaction between neighboring Si atoms, leads to the transition. The proposed mechanical control of the electronic properties will widen the application of the silicene in Si-based nanotechnology.

The effect of cation doping on spinel LiMn2O4: a first-principles investigation

The effect of the cation doping on the electronic structure of spinel LiMyMn2-yO4 (M = Cr, Mn, Fe, Co and Ni) has been calculated by first-principles. Our calculation shows that new M-3d bands emerge in the density of states compared with that in LiMn2O4. Simultaneously, the new O-2p bands appear accordingly in almost the same energy range around the Fermi energy owing to the M-3d/O-2p interaction. It is found that the appearance of new O-2p bands in the lower energy position results in a higher intercalation voltage. Consequently, the origin of higher intercalation voltage in LiMyMn2-yO4 can be ascribed to the lower O-2p level introduced by the doping cation M

W6+ & Br− codoped Li4Ti5O12 anode with super rate performance for Li-ion batteries

We report a novel Li4Ti5−xWxO12−xBrx (x = 0.025, 0.050 and 0.100) anode material simultaneously doped with W6+ and Br− ions prepared by a simple solid-state reaction in air, aiming to significantly improve electrical conductivity of Li4Ti5O12. Our theoretical calculation predicts that codoping with W6+ on the Ti4+ site and Br− on the O2− site can remarkably narrow down the band gap, and thus facilitate the electron transport in the lattice of LTO. The comparative experiments prove that W & Br-codoped LTO exhibits higher electrical conductivity compared with undoped LTO as expected, thus leading to improved rate capability and specific capacity. Particularly, Li4Ti5−xWxO12−xBrx (x = 0.05) exhibits the best rate capability and cycling stability with an outstanding capacity retention of 88.7% even at 10 C rate after 1000 cycles. This codoping strategy with high valence transition metal and halide ions holds promise to be applied to other insulating cathode materials suffering from inferior electrical conductivity.

Is silicene stable in O2???First-principles study of O2 dissociation and O2-dissociation?induced oxygen atoms adsorption on free-standing silicene

The stability of free-standing silicene in O2 is an open question. In this letter, the O2 dissociation and O2-dissociation–induced O atoms adsorption on free-standing silicene are studied by using first-principles calculations. Our results show that the O2 molecule dissociates on the free-standing silicene surface easily from both the thermodynamic and kinetic points of view, which is different from the case of graphene. The dissociation reaction is an exothermic process, and the dissociated O atoms form strong bonds with Si atoms, which lowers the energy of the system substantially. On the other hand, the dissociation reaction occurs spontaneously on the free-standing silicene without overcoming any energy barrier. Furthermore, the migration and desorption of O atoms are relatively difficult under room temperature due to the strong Si-O bonds in the O-adsorbed silicene, which is in favor of forming silicon oxides. Our results provide convictive evidence to show that free-standing silicene is unstable in O2.

Comparison of the stability of free-standing silicene and hydrogenated silicene in oxygen: a first principles investigation.

The stability of free-standing silicene in oxygen is worthwhile discussing. In this letter, the oxygen adsorption and dissociation on free-standing silicene is studied using first principles. It is found that free-standing silicene is not stable in oxygen because O2 molecules can be easily adsorbed and dissociated into O atoms on a silicene surface without overcoming any energy barrier. Moreover, dissociated oxygen atoms are difficult to migrate on and desorb from silicene surfaces, leading to the formation of Si-O compounds. To enhance the stability of free-standing silicene in oxygen, fully hydrogenated silicene is used as a stabiliser. Interestingly, compared with no energy barrier on pristine silicene, there are two minor energy barriers of O2 molecule adsorption and dissociation on fully hydrogenated silicene, indicating that hydrogenated silicene has higher stability than free-standing silicene in oxygen. However, once the O2 molecule dissociates into two O atoms on hydrogenated silicene, desorption of O atoms will be very difficult due to its high energy barrier. This work will be helpful to understand the detail of O2 molecule dissociation and dissociation-induced O atoms adsorption on free-standing and hydrogenated silicene in oxygen and will be useful to the application of silicene.