Heusler合金Li2AlX(X=Ga，In)的晶格动力学和电子特性

本文计算了Heusler合金Li₂AlGa和Li₂AlIn的晶格参数、体积模量、体积模量的一阶导数、 电子能带结构、声子色散曲线和声子态密度,并与密度泛函理论中的广义梯度近似计算结果进行比较. 计算的晶格参数与文献有很好的一致性. 两个Heusler合金的电子能带结构表明它们是半金属结构. 并利用声子色散曲线和声子密度图研究Heusler合金晶格动力学. Li₂AlGa和Li₂AlIn Heusler合金在基态呈现动力学稳定.     

Coherence and spike death induced by bounded noise and delayed feedback in an excitable system

The first-principles total energy calculations with the generalised gradient approximation and the plane-wave pseudopotential method have been employed to investigate the structural, electronic and dynamical properties of Li₂TiO₃ (lithium titanate). The atomic structure is fully relaxed, and the structural parameters are found to differ by less than 1% from the experimental data. The indirect band-gap with 3.49 eV is predicted from the band structure calculations of this compound. The calculated phonon frequencies at the ??-point for the Raman-active and the infrared-active modes are presented and assigned. The phonon dispersion curves are also calculated along high symmetry lines in the Brillouin zone (BZ). Furthermore, the thermodynamic functions have been worked out using the corresponding phonon density of states, and the results indicate that they are in good agreement with available experimental values.     

Lattice dynamical study of alkali metals: An unified approach based on CGW model

The phonon dispersion relations for lithium, sodium, potassium, rubidium and cesium along the principal symmetry directions as well as their lattice specific heats have been deduced using Clark, Gazis and Wallis angular force model. This model which conforms to the translational symmetry of the lattice, reproduces the observed crossover in lithium along [ζ00] direction at ζ = 0·49, without producing any crossovers in other alkali metals. Besides, the theoretical dispersion curves of all alkali metals are in excellent agreement with the corresponding experimental or homologous dispersion relations and theirϑ _D values compare well with the experimental values over a wide temperature range. It is shown that the strength of electron-ion interactions plays a significant role in the success of any unified lattice dynamical study of alkali metals while the three-body interactions of thecgw model do not. The importance of umklapp processes, failure of the earlier models to produce a crossover and the experimentalϑ _D-T curve in lithium as well as the apparent variation in the nature and range of atomic interactions of alkali metals are discussed.     

Lattice dynamical study of body-centred tetragonal indium

The phonon dispersion curves, phonon frequency distribution function as well as the lattice specific heat of body-centred tetragonal indium have been deduced using a lattice dynamical model which includes central, angular and volume forces. Six elastic constants, four zone boundary frequencies and an equilibrium condition were used in the evaluation of the force constants. It is shown that this model is elastically consistent and satisfies the symmetry requirements of the lattice, the phonon frequencies of indium deduced from it are in very good agreement with the experimental values of Reichardt and Smith and the theoretical values of Garrett and Swihart, and theθ _D values compare well with the experimental values over a wide temperature range. The apparent discrepancies in the phonon dispersion curves and theθ _D-T curves obtained from deficient models, importance of umklapp processes and the significance of angular forces in the lattice dynamical models are discussed.     

Phonon Dispersion in Equiatomic Li-Based Binary Alloys

The computations of the phonon dispersion curves （PDC） of four equiatomic Li-based binary alloys, namely Li0.5Na0.5, Li0.5K0.5, Li0.5Rb0.5 and Li0.5Cs0.5, to second order in the local model potential is discussed in terms of the real-space sum of Born von Karman central force constants. Instead of the concentration average of the force constants of metallic Li, Na, K, Rb and Cs, the pseudo-alloy atom （PAA） is adopted to compute directly the force constants of four equiatomic Li-based binary alloys. The exchange and correlation functions due to Hartree （H） and Ichimaru-Utsumi （IU） are used to investigate the influence of screening effects. The phonon frequencies of four equiatomic Li-based binary alloys in the longitudinal branch are more sensitive to the exchange and correlation effects in comparison with the transverse branches. However, the frequencies in the longitudinal branch are suppressed due to IU-screening function than the frequencies due to static H-screening function.     

Polaron states and migration in F-doped Li2MnO3

Li₂MnO₃ compound is one essential component of the Li-rich solid solution cathode material (mLi₂MnO₃⋅nLiMO₂, M = Mn, Ni, Co, etc.) for lithium ion batteries. The intrinsic insulating nature of the electronic structure determines that the electronic conductivity of the Li₂MnO₃ compound is low, which is unfavorable to the fast charge/discharge performance of the battery. In this paper, we demonstrate that F doping can create polaron states in the Li₂MnO₃ lattice from first principles calculations. The small polaron migration energy barrier in the Li₂MnO₃ lattice is about 0.27 eV. It is also observed that the polaron state is strongly trapped by the F atom, which decreases the efficiency of the doping enhanced electronic conductivity.     

First-principles study of high pressure phase transformations in Li<Subscript>3</Subscript>N

The lattice dynamics of lithium nitride (Li₃N) under high pressure are extensively investigated to probe its phase transformations by using the pseudopotential plane-wave method within the density functional theory. A new second order α↦α^′-Li₃N phase transition is identified for the first time. The newly proposed α^′-phase possesses a hexagonal symmetry with four ions in the unit cell having a space group of P-3m1. Further enthalpy and phonon calculations support the existence of this phase, which stabilizes in a narrow pressure range of 2.8 – 3.6 GPa at zero temperature. Upon further compression, transitions to denser packed phases of β-and γ-Li₃N are typical first order. The analysis of the electronic densities of states suggests that all the high pressure modifications of Li₃N are insulators and, interestingly, the typical behavior of compression is to broaden the band gap.     

Synthesis and redox behavior of a new polyanion compound, Li2Co2(MoO4)3, as 4 V class positive electrode material for lithium batteries

A new material, Li₂Co₂(MoO₄)₃, belonging to NASICON type polyanion family was synthesized by means of a low temperature soft-combustion method using glycine as a soft combustion fuel. The annealed product, Li₂Co₂(MoO₄)₃, was found to exhibit a single phase structure as confirmed by XRD and crystallized in an orthorhombic structure (space group Pnma) with lattice parametersa=5.086(1) Å,b=10.484(2) Å and c=17.606(2) Å. The electronic state of each element present in the new material was confirmed by X-ray photoelectron spectroscopic (XPS) analysis. The stoichiometry of the synthesized product was determined by the metal analysis using inductively coupled plasma (ICP-AES) technique. The microstructural analysis by means of SEM revealed cylindrical fiber-like grains. Electrochemistry of the new material was demonstrated by extraction/insertion process of Li⁺ in lithium batteries. Galvanostatic charge/discharge profiles revealed a reversible discharge capacity of ～ 55 mAh/g over the potential window of 4.9 - 1.5 V.     

Infrared studies of lithium intercalation in the FePS3 and NiPS3 layer-type compounds

Infrared spectra (700-30 cm^-1) of several lithium intercalates (chemically prepared) Li_xMPS₃, with M=Fe, Ni and 0<x<1.5, have been recorded and compared with those known for the corresponding host lattices. These lithium intercalates are mainly characterized by new absorption bands at 336 cm^-1 and 310 cm^-1 for the iron and nickel compounds, respectively. These bands assigned to lithium vibrations increase progressively with lithium content : it is concluded that Li⁺ ions are more likely to occupy the 2d and 4h “octahedral” sites in the gaps. In addition, the spectra of the nickel derivatives reveal some geometric distortion within the layers and a progressive strengthening of the Ni-S interactions. These results are correlated to the best energy yields obtained in NiPS₃/lithium batteries.     

Crystal structure analysis of novel complex hydrides formed by the combination of LiBH4 and LiNH2

Combination of LiBH₄ and LiNH₂ by ball milling forms the series of novel complex hydrides Li₂BNH₆, Li₃BN₂H₈ and Li₄BN₃H₁₀, depending on the combination ratios. The crystal structure of Li₄BN₃H₁₀ analyzed by synchrotron X-raydiffraction measurements is determined to be a cubic system (space group: I2₁3) with the lattice constant of a=10.673(2)Å. It should be emphasized that Li₄BN₃H₁₀ is an ionic crystal which is composed of a lithium cation Li⁺ and two different kinds of the complex anion [BH₄]^- and [NH₂]^-. These anions are located in the vertex and face-center of the cubic sub-lattice, and the lithium cation Li⁺ in the interstitial site between the anions, respectively. The other series of complex hydrides, Li₂BNH₆ and Li₃BN₂H₈, are also predicted to possess similar structures composed of a lithium cation Li⁺ and two different kinds of the complex anion [BH₄]^- and [NH₂]^-.     

Structural and electronic properties of the Li-ion battery cathode material LixCoSiO4

The first-principles density functional theory has been employed to study the structural and electronic properties of Li_xCoSiO₄. The lattice stability of Li_xCoSiO₄ during the lithiation–delithiation process is discussed. The changes in the electronic structures of Li_xCoSiO₄ during the deintercalation of Li ions are also probed. It is found that Li₂CoSiO₄ reacts reversibly with 1 Li⁺ at an average voltage of 4.1 V versus a lithium anode. The computational results indicate that Li₂CoSiO₄ material is a potential candidate for high-capacity cathode for advanced lithium ion batteries.     

石墨/Li（Ni1/3Co1/3Mn1/3）O2电池充放电过程中电极材料的XRD研究

利用XRD系统地研究了石墨/Li（Ni_1/3Co_1/3Mn_1/3）O₂ 18650型锂离子电池充放电过程中正负极活性材料的晶体结构和微结构的变化.已观测到,由于Li原子的脱嵌,使得LiMO₂点阵参数a缩小,c增大,微应变增大,衍射强度比I₁₀₄/I₁₀₁和I₀₁₂/I₁₀₁降低;此外,由于Li原子的嵌入,2H-石墨的点阵参数a和c,以及微应变ε和堆垛无序度P都增加.同时,讨论了活性材料Li（Ni_1/3Co_1/3Mn_1/3）O₂和石墨在电池充放电过程中的嵌脱锂的物理机理.在充电时,正极Li（Ni_1/3Co_1/3Mn_1/3）O₂中处于（000）位的Li原子优先脱离晶体点阵,继后才是位于（2/3 1/3 1/3）和（1/3 2/3 2/3）位的Li原子离开点阵.锂嵌入石墨,优先进入碳原子六方网格面间的间隙位置,当负极的堆垛无序度达到一定值后,3R相逐渐析出.当电池满充或过充时,在六方石墨中形成LiC₁₂和LiC₆相.放电时,与上述过程相反,但并非是完全可逆的. 关键词： 锂离子电池 微结构 X射线衍射 嵌脱锂物理机理     

Lattice dynamics of lithium oxide

Li₂O finds several important technological applications, as it is used in solid-state batteries, can be used as a blanket breeding material in nuclear fusion reactors, etc. Li₂O exhibits a fast ion phase, characterized by a thermally induced dynamic disorder in the anionic sub-lattice of Li⁺, at elevated temperatures around 1200 K. We have carried out lattice-dynamical calculations of Li₂O using a shell model in the quasi-harmonic approximation. The calculated phonon frequencies are in excellent agreement with the reported inelastic neutron scattering data. Thermal expansion, specific heat, elastic constants and equation of state have also been calculated which are in good agreement with the available experimental data.     

Electrochemical properties and structures of the mixed-valence lithium cuprates Li3Cu2O4 and Li2NaCu2O4

The electrochemical performances of Li₃Cu₂O₄ and Li₂NaCu₂O₄ as cathode materials in lithium coin type batteries have been studied. In Li₃Cu₂O₄, the copper was oxidised to the III level when cycling. The replacement of the lithium by the sodium ions in the octahedral sites in Li₂NaCu₂O₄ might have an effect on the pathway of the lithium ions during the (de)intercalations. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Fabrication and electrochemical investigation of Li<Subscript>0.5</Subscript>TiO<Subscript>2</Subscript> as anode materials for lithium ion batteries

Hexagonal and cubic Li_0.5TiO₂ particles have been fabricated through magnesiothermic reduction of Li₂TiO₃ particles in a temperature range of 600 to 640 °C. The prolonged reduction time results in lattice transition from hexagonal to cubic structure of Li_0.5TiO₂. Their microstructures, valance state, chemical composition, as well as electrochemical performance as anode candidates for lithium ion batteries have been characterized and evaluated. The hexagonal Li_0.5TiO₂ exhibits better electrochemical activity compared with the cubic one. Further, the carbon-coated hexagonal Li_0.5TiO₂ displays improved electrochemical performance with initial reversible capacity of 176.6 mAh g^?1 and excellent cyclic behavior except capacity fading in the initial 10 cycles, which demonstrate a novel anode candidate for long lifetime lithium ion batteries.     

First-principles study of lattice dynamics of LiFePO4

Lattice dynamics of lithium iron orthophosphate (LiFePO₄) isostructural with olivine have been investigated using the first-principles calculations taking into account the on-site Coulomb interaction within the GGA + U scheme. Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves are calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives a convincing evidence for the one-dimensional Li migration tunnel along the [010] direction in LiFePO₄, which has been proposed by other theoretical calculations and experimental observation. The calculated phonon frequencies at the Γ point of the Brillouin zone show good agreement with the available experimental observations.     

Lithium ordering and NMR linewidth in Li0.33TiS2

The low temperature ⁷Li linewidths and second moments have been analyzed for Li_0.33 TiS₂ and Li_0.97 TiS₂. Moment calculations demonstrate that the linewidth is a sensitive indicator of order on a two dimensional lattice of dipoles. The calculated dipolar second moment is not consistent with a simple √3 a_o superlattice in Li_0.33 TiS₂, but is consistent with a random arrangement of lithium ions and several more complex superlattices recently proposed by Kanamori and Kaburagi.     

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

Enhanced rate performance of lithium titanium oxide anode material by bromine doping

Br-doped lithium titanium oxide (Li₄Ti₅O₁₂) particles in the form of Li₄Ti₅Br _x O_12-x (x?=?0, 0.1, 0.2, 0.3, 0.4) are synthesized via a simple liquid deposition reaction, followed by a high-temperature treatment. The effects of bromine (Br) doping on the structures and electrochemical properties of Li₄Ti₅O₁₂ are extensively studied. It is found that Br atoms can enter the lattice structure and enlarge the lattice parameters of Li₄Ti₅O₁₂. Although Br doping has not changed the phase composition, obvious effects on the particle’s morphology and size have been observed. Electrochemical test results indicate that the rate capability of Li₄Ti₅O₁₂ has been evidently improved by Br doping at an appropriate concentration. The as-synthesized Li₄Ti₅O_11.8Br_0.2 electrode presents much higher discharge capacity and better cycle stability than that of the other electrodes. The greatly enhanced electrochemical performance of Li₄Ti₅O_11.8Br_0.2 may be attributed to the improved dispersion of nanoparticles and increased electrical conductivity.