Li-Li Interaction Research Articles

Disentangling Cation and Anion Dynamics in Li3PS4 Solid Electrolytes.

A prerequisite for the realization of solid-state batteries is the development of highly conductive solid electrolytes. Li3PS4 is the archetypal member of the highly promising thiophosphate family of Li-ion conductors. Despite a multitude of investigations into this material, the underlying atomic-scale features governing the roles of and the relationships between cation and anion dynamics, in its various temperature-dependent polymorphs, are yet to be fully resolved. On this basis, we provide a comprehensive molecular dynamics study to probe the fundamental mechanisms underpinning fast Li-ion diffusion in this important solid electrolyte material. We first determine the Li-ion diffusion coefficients and corresponding activation energies in the temperature-dependent γ, β, and α polymorphs of Li3PS4 and relate them to the structural and chemical characteristics of each polymorph. The roles that both cation correlation and anion libration play in enhancing the Li-ion dynamics in Li3PS4 are then isolated and revealed. For γ- and β-Li3PS4, our simulations confirm that the interatomic Li-Li interaction is pivotal in determining (and restricting) their Li-ion diffusion. For α-Li3PS4, we quantify the significant role of Li-Li correlation and anion dynamics in dominating Li-ion transport in this polymorph for the first time. The fundamental understanding and analysis presented herein is expected to be highly applicable to other solid electrolytes where the interplay between cation and anion dynamics is crucial to enhancing ion transport.

Investigation of Thermodynamic Nonideality of Solid Li-ion Electrodes Using Concentrated Solution Theory

Li-Ion batteries (LIBs) is poised to dominate the energy storage market of electric vehicles (EVs) and renewable-energy-integrated grids over the next few decades. To address safety and lifetime concerns of the technology, significant efforts have been devoted to advancing online battery diagnostic tools and evolving charging/discharging protocols to enhance their stability and elongate their lifetime, and both have been found to be highly amenable to physics-based models (PBMs). With the continuous improvements and breakthroughs of computer technology, PBMs in substitute of equivalent circuit models (ECMs) have been employed to produce high-fidelity, low-computational-cost reduced-order-models (ROMs) for battery management system (BMS), despite the fact that simplifications have regularly been made. For example, in a general PBM of LIBs, Li-ion diffusion is governed by Fick’s second law of diffusion in a dilute solution (Fig. 1), which neglected the thermodynamic nonideality of a solid active material which experiences large concentration variation in electrochemical processes. Though the above fact likely indicates that such PBMs contain some battery physics to represent the key electrochemical processes, it does not exclude the possibility that important correlations could be associated with physics that have been neglected. For example, although both ks (solid/liquid interface reaction rate) and D (diffusivity of Li-ion in a solid phase) exhibited correlations with battery State-Of-Charge (SOC), especially at the end of discharge, they have been assumed to be constants over the entire range of SOC. In fact, their values reported from different researchers show variation spanning over 2 to 5 order of magnitude. With this large variation, both battery SOC and State-Of-Health (SOH) will remain unpredictable, which means that ultimately the battery management and diagnosis remains uncertain. Numerous theoretical and experimental studies support the inevitable Li-Li interactions when Li-ion deintercalation/intercalation from/into the active materials, especially with a sufficiently large concentration. This Li-Li interaction introduces thermodynamic nonideality which could not be described by the Fick’s second law of diffusion. Consequently, critical electrochemical kinetic and transport properties such as ks and D will have a strong correlation with the SOC. In this research, we will try to understand the nonideality of a solid Li-ion solution by the concentrated solution theory, as shown in Fig.1. We will use nonequilibrium thermodynamics in a concentrated solution to correlate ks/D with the activity coefficient of the solid solution γ and elaborate new governing equations for D and ks. Then, utilize the new correlations to process experimental data tested by EIS method and to regress the experimental activity coefficient γ. Furthermore, a methodology in determining the most appropriate constants for ks and D with a minimum discrepancy for a full charge/discharge cycle will be provided. Figure 1. (a) Inconsistency in KsS and D values between dilute solution model prediction and experimental results; (b) Ks and D comparison between dilute solution theory and concentrated solution theory; (c) Experimental and model prediction based on concentrated solution theory. Figure 1

Li adsorption on monolayer and bilayer MoS2 as an ideal substrate for hydrogen storage**Project supported by the National Key Basic Research Program of China (Grant No. 2012CB932304), the National Natural Science Foundation of China (Grant No. 21763007), the Innovation Team Foundation of the Education Department of Guizhou Province, China (Grant No. [2014]35), and the Key Laboratory of Low Dimensional Condensed

Based on the first-principles plane wave calculations, we show that Li adsorbed on monolayer and bilayer MoS2 forming a uniform and stable coverage can serve as a high-capacity hydrogen storage medium, and Li-coated MoS2 can be recycled by operations at room temperature due to Li having strength binding, big separation and is stable against clustering. The full Li coverage MoS2 system (2*2 hexagonal MoS2 supercell) can reach up to eight H2 molecules on every side, corresponding to the gravimetric density of hydrogen storage up to 4.8 wt% and 2.5 wt% in monolayer and bilayer MoS2, respectively. The adsorption energies of hydrogen molecules are in the range of 0.10eV/H2–0.25 eV/H2, which are acceptable for reversible H2 adsorption/desorption near ambient temperature. In addition, compared with light metals decorated low dimension carbon-based materials, the sandwiched structure of MoS2 exhibits the greatly enhanced binding stability of Li atoms as well as slightly decreased Li-Li interaction and thus avoids the problem of metal clustering. It is interesting to note that the Li atom apart from the electrostatic interaction, acts as a bridge of hybridization between the S atoms of MoS2 and adsorbed H2 molecules. The encouraging results show that such light metals decorated with MoS2 have great potential in developing high performance hydrogen storage materials.

Interaction and concerted diffusion of lithium in a (5,5) carbon nanotube

The interaction and diffusion of lithium atoms in a (5,5) carbon nanotube is studied using density-functional theory. The Li-nanotube interaction perpendicular to the tube axis for a single Li inside and outside the tube is calculated and compared with the Li-graphene interaction obtained using the same technique. Both interactions are similar in the repulsive region but exhibit differences in their attractive part. Nevertheless, they can be described using a common parametrization. The Li-Li interaction is calculated as a function of their separation inside the tube. This interaction is similar to a screened repulsive Coulomb potential at small separations. However, at larger separations, the Li-Li interaction does not vanish and shows residual oscillations. This repulsive long-ranged interaction favors concerted diffusion of many Li atoms compared to the independent diffusion of individual Li inside the tube.

Cyclic voltammetry studies on 4 V and 5 V plateaus of non-stoichiometric spinel Li 1+ xMn 2− yO 4

Li 1+ x Mn 2− y O 4 spinels with various Li/Mn ratios were synthesized by a solid-state reaction. By X-ray diffraction analysis, Li 2MnO 3 was detected as a second phase with increasing the Li/Mn ratio; and the role of Li 2MnO 3 in Li 1+ x Mn 2− y O 4 spinel was discussed. A slow scanning cyclic voltammetry(CV) at the rate of 0.1 mV/s was adopted to characterize the evolutions of 4 V and 5 V plateaus of Li 1-xMn 2− y O 4 spinels. An additional Li + insertion in 4 V region was observed in both Li-lack and Li-rich spinels at 3.95 V, which is different from the general Li + insertion with weak Li-Li interaction and strong Li-Li interaction; and this plateau disappeared in the subsequent cycles. The 4.4 V/3.8 V plateaus correspondent to Li + insertion and extraction of Li 2MnO 3 were discussed, and these plateaus have a high reversibility with cycling. The 5 V plateau was found only in the Li-rich samples, and this plateau has a tendency to emerge at higher voltage region with increasing Li/Mn ratio.

Large-scale computer modelling of Li impurities in KTaO 3 and K 1–xLi xTa 1–yNb yO 3 perovskite solid solutions

The magnitudes of off-center Li displacements and the relaxation energies related to reorientation of Li are calculated in ABO 3 perovskites using the semi-empirical Hartree-Fock-based method of the Intermediate Neglect of Differential Overlap (INDO). The spatial extent of a lattice relaxation around Li impurities and contributions from different neighbours to the relaxation energy are discussed. We have applied the INDO method for the study of interaction between Li impurities in KTaO 3. The Li-Li interaction energy as a function of the Li-Li distance and orientation are calculated, including the lattice relaxation around Li-Li impurities. According to our calculations, the Li-Li interaction energies are smaller and less long-range than those estimated from earlier shell model calculations. Lastly, we discuss theoretical modelling of KTaO 3 perovskites with a small amount of Li and simultaneously diluted by Nb (K 1–xLi xTa yNb 1–yO 3, KLTN).

Semi-empirical hartree-fock calculations for interacting Li impurities in KTaO3

Equilibrium off-center displacements of two Li impurities substituting neighboring K sites in KTaO3, the relaxation pattern of O ions around the impurities and the Li-Li interaction energies in fixed and relaxed O surrounding have been calculated using the semi-empirical INDO method. The Li off-center displacement is 0.6 – 0.7 Å in all cases: the displacements of O nearest neighbors to Li are up to 0.06 Å, and the displacement of more distant atoms negligibly small, within the calculational accuracy. The Li-Li interaction energy is positive, revealing the deviations from the dipole-dipole interaction in a polarized crystal, and strongly enhanced due to the O relaxation.

Low-temperature phase in LixK1-xTaO3

Abstract Lithium, substituting K in the perovskite KTaO3, creates a local electric dipole due to its off-center position with respect to the cubic site. Inter-action between these moments leads to a low-temperature phase which is still controversial, since it reveals signatures of both glass-like and long range order behavior. This work covers recent theoretical investigations of the effective Li-Li interaction and the nature of the low-temperature phase in Li x K1-x TaO3, performed in the framework of a non-linear shell model.

Suppression of staging in lithium-intercalated carbon by disorder in the host.

We report electrochemical and x-ray-diffraction studies of the intercalation of lithium in graphite and in disordered carbons. The phase diagram of electrochemically intercalated graphite agrees well with previous work on samples prepared by chemical methods. The well-known staged phases present in intercalated graphite are absent in intercalated petroleum coke. Furthermore, the voltage V(x) of Li/${\mathrm{Li}}_{\mathit{x}}$${\mathrm{C}}_{6}$ cells differs greatly when graphite or coke is used as the host. By heating coke to successively higher temperatures, we are able to increase the graphitization or crystalline order of the host in a continuous fashion and study the effect of this variation on the phase diagram of ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{C}}_{6}$ and on V(x). We find that staged phases are suppressed at room temperature for hosts less ordered than a ``critical disorder.'' A lattice-gas model with random site energies is used to model the effects of host disorder and qualitatively explains the suppression of staged phases and the changes in V(x) with increasing disorder in the host. For a rectangular ``density of sites,'' staged phases are suppressed when the width of the site energy distribution is greater than the magnitude of the mean-field attractive Li-Li interaction, which causes island growth and staging in intercalated graphite.

Lattice dynamics of lithium-intercalated iron phosphorus trisulphide

We review the recent investigations on the lattice dynamics of lithium-intercalated FePS 3. These studies represent an extension of those made for the pure material and have been performed by means of a force-constant model generated by a set of short-range two-body potentials. The intercalated phases have been investigated for the three stoichiometric compositions Li 0.5FePS 3, LiFePS 3 and Li 1.5FePS 3 with the aim of analysing the evolution of the host lattice normal modes as a function of the concentration, and finding the dispersion of the new phonon branches induced by lithium. The above special values of lithium concentration have been chosen because the size of the unit cell remains the same as in the host material. The interaction between lithium and sulphur includes only nearest neighbours and is also described by a two-body potential. The Li-Li interaction is described by a Born-Mayer potential. The choice of this simplified model is dictated by the limited number of experimental data: these are the Raman and infrared data for the host material and only the infrared data for the intercalated phases. The force constants of lithium are fitted to these data and the phonon dispersion curves are calculated along the symmetry directions of the Brillouin zone. A spectroscopic method for monitoring lithium migration in the host material is proposed.

Theory of lithium ordering in LixTiS2

Real space renormalization group and Monte Carlo techniques are used to study the statistical mechanics of a lattice gas model of Li ordering in an intercalation battery. The dependence of the voltage on the state of charge of the battery is related to the number and types of sites available to the Li ions and to the strength and distance dependence of the Li-Li interaction. Several models are discussed and compared with the observed behaviour of Li xTiS 2.

Re-evaluation of some lithium dipole properties and dipole-dipole and dipole-quadrupole van der Waals constants

Our previous one-centre pseudo-state results for some ground-state Li atom dipole oscillator strength sums, the dipole-dipole dispersion energy constants for the Li-Li, Li-H and Li-He ground-state interactions and the dipole-quadrupole dispersion energy constant for the Li-Li interaction are re-evaluated in the light of a more reliable selection for the resonance dipole oscillator strength of the Li atom. These and some of our previous lithium pseudo-state results are compared with corresponding literature values.