Shi Si-Qi

First Principles Study on NaxLi1−xFePO4 As Cathode Material for Rechargeable Lithium Batteries

The electronic structure and ionic dynamic properties of pure and Na doped (Li site) LiFePO4 have been investigated by first-principles calculations. The band gap of the Na doped material is much narrow than that of the undoped one, indicating of better electronic conductive properties. First-principles based molecular dynamic simulations have been performed to examine the migration energy barriers for the Li ion diffusion. The results shown that the energy barriers for Li diffusion decreased a little along the one-dimensional diffusion pathway, indicating that the ionic conductive property is also improved, as compared with the high valance doping (such as Cr) cases.

Structural and magnetic properties of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4−δ spinels: A first-principles study

Structural and magnetic properties of LiNi0.5Mn1.5O4 and LiNi0.5Mn1.5O4−δ are investigated using density-functional theory calculations. Results indicate that nonstoichiometric LiNi0.5Mn1.5O4−δ and stoichiometric LiNi0.5Mn1.5O4 exhibit two different structures, i.e., the face-centred cubic (Fd-3m) and primitive, or simple, cubic (P4332) space groups, respectively. It is found that the magnetic ground state of LiNi0.5Mn1.5O4 (P4332 and Fd-3m) is a ferrimagnetic state in which the Ni and Mn sublattices are ferromagnetically ordered along the [110] direction whereas they are antiferromagnetic with respect to each other. We demonstrate that it is the presence of an O-vacancy in LiNi0.5Mn1.5O4−δ with the Fd-3m space group that results in its superior electronic conductivity compared with LiNi0.5Mn1.5O4 with the P4332 space group.

First-principles investigation of the electronic structure and magnetism of eskolaite

The electronic structure and magnetism of eskolaite are studied by using first-principles calculations where the on-site Coulomb interaction and the exchange interaction are taken into account and the LSDA+U method is used.The calculated energies of magnetic configurations are very well fitted by the Heisenberg Hamiltonian with interactions in five neighbour shells; interaction with two nearest neighbours is found to be dominant. The Neel temperature is calculated in the spin-3/2 pair-cluster approximation. It is found that the measurements are in good agreement with for the values of U and J that are close to those obtained within the constrained occupation method.The band gap is of the Mott-Hubbard type.

Temperature-Dependent Dynamic Properties of LixMn2O4 in Monte Carlo Simulations

Monte Carlo (MC) simulations are used to simulate the voltage profile and the ionic conductivity s of Li ions in LixMn2O4 and its dependence on the lithium concentration x. The open circuit potential shows clearly the two plateaus in the charge/discharge curve, which agrees well with the experimental results. The two plateaus become more and more steep when the temperature is increased. The simulated ionic conductivity shows an M-shaped curve in the plot of ionic conductivity σ versus x when the simulation temperature is low. Interestingly, the minimum valley, which lies at the middle single-phase area near x = 0.5, disappears gradually when the temperature increases to 453 K.

溶胶-凝胶法制备钠离子固态电解质Na 3 Zr 2 Si 2 PO 12 及其电导性能研究

采用溶胶-凝胶法, 探索了合成纯相Na 3 Zr 2 Si 2 PO 12 的条件, XRD分析表明前驱体中Na和P 均过量10%时, 在1050℃下烧结得到了纯相的Na 3 Zr 2 Si 2 PO 12 , 而P不过量时, 得到的产物中含有少量的ZrO 2 杂质。电化学阻抗谱测试表明Na和P 均过量10%时, 烧结得到的陶瓷片室温离子电导率达到了5.4×10 -4 S/cm, 比P不过量时烧结得到样品的离子电导率(室温时为3.7×10 -4 S/cm)要高。进一步分析可知高温下P的挥发造成ZrO 2 杂质相的析出, 从而使得离子电导率降低。与固相法相比, 溶胶-凝胶法需要的烧结温度更低, 且制得的样品的离子传导性更好。NASICON-structured Na3Zr2Si2PO12 was synthesized by a Sol-Gel approach. Phase-pure samples were successfully sintered at 1050 degrees C when adding 10% excessive Na and P in the precursors, while a small amount of ZrO2 impurity was detected without adding excessive phosphorus. Electrochemical impedance spectrum tests indicate that the ionic conductivity of the former is as high as 5.4x10(-4) S/cm at room temperature, which is higher than that of samples prepared from the precursors without adding excessive phosphorus (3.7x10(-4) S/cm). Further analysis reveals that the evaporation of phosphorus at high temperature would cause the formation of ZrO2 impurity in the samples, leading to a lower ionic conductivity. Compared with solid state reaction approach, samples with enhanced ionic conductivity can be obtained at a rather lower temperature by Sol-Gel synthesis.采用溶胶-凝胶法, 探索了合成纯相Na 3 Zr 2 Si 2 PO 12 的条件, XRD分析表明前驱体中Na和P 均过量10%时, 在1050℃下烧结得到了纯相的Na 3 Zr 2 Si 2 PO 12 , 而P不过量时, 得到的产物中含有少量的ZrO 2 杂质。电化学阻抗谱测试表明Na和P 均过量10%时, 烧结得到的陶瓷片室温离子电导率达到了5.4×10 -4 S/cm, 比P不过量时烧结得到样品的离子电导率(室温时为3.7×10 -4 S/cm)要高。进一步分析可知高温下P的挥发造成ZrO 2 杂质相的析出, 从而使得离子电导率降低。与固相法相比, 溶胶-凝胶法需要的烧结温度更低, 且制得的样品的离子传导性更好。