Predicted structure, thermo-mechanical properties and Li ion transport in LiAlF4 glass

• Published in: Journal of non-crystalline solids Vol. 358; no. 16; pp. 1917 - 1923
• Main Authors: Stechert, T.R., Rushton, M.J.D., Grimes, R.W., Dillon, A.C.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier B.V 15-08-2012; Elsevier
• Subjects: ACTIVATION ENERGY; Applied sciences; ATOMS; Battery; BOND ANGLE; CHAINS; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; COATINGS; Condensed matter: structure, mechanical and thermal properties; CORRELATION FUNCTIONS; DIFFUSION; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; ELECTRODES; ENERGY STORAGE; Exact sciences and technology; FLUORINE; FLUORINE IONS; GLASS; Glass transition; iconic conductivity; Ionic conductivity; Lithium diffusion; Lithium ion batteries; LITHIUM IONS; MATERIALS SCIENCE; Mathematical models; Molecular dynamics; Molecular structure; Movement; Networks; Physics; STATISTICS; Structure and morphology; thickness; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties); THERMAL EXPANSION; Thin film structure and morphology; TOPOLOGY; TRANSITION TEMPERATURE; TRANSPORT; Glass transition; Lithium ion batteries; Molecular dynamics; Lithium diffusion; Ionic conductivity; Pair correlation function; Molten state; Fluorine; Bond angle; Molecular dynamics method; Glass; Topology; Glass transition temperature; Lithium battery; Thermal expansion; Atomic arrangement; Structural model; Property structure relationship; Trajectory; Diffusion coefficient; Diffusion; Activation energy; Protective coatings; Amorphous state structure
• ISSN: 0022-3093; 1873-4812
• DOI: 10.1016/j.jnoncrysol.2012.05.044

Materials with the LiAlF4 composition are of interest as protective electrode coatings in Li ion battery applications due to their high cationic conductivity. Here classical molecular dynamics calculations are used to produce amorphous model structures by simulating a quench from the molten state. These are analysed in terms of their individual pair correlation functions and atomic coordination environments. This indicates that amorphous LiAlF4 is formed of a network of corner sharing AlF6 octahedra. Li ions are distributed within this network, primarily associated with non-bridging fluorine atoms. The nature of the octahedral network is further analysed through intra‐ and interpolyhedral bond angle distributions and the relative populations of bridging and non-bridging fluorine ions are calculated. Network topology is considered through the use of ring statistics, which indicates that, although topologically well connected, LiAlF4 contains an appreciable number of corner‐linked branch‐like AlF6 chains. Thermal expansion values are determined above and below the predicted glass transition temperature of 1340K. Finally, movement of Li ions within the network is examined with predictions of the mean squared displacements, diffusion coefficients and Li ion activation energy. Different regimes for lithium ion movement are identified, with both diffusive and sessile Li ions observed. For migrating ions, a typical trajectory is illustrated and discussed in terms of a hopping mechanism for Li transport.
► Simulation of LiAlF4 glass structure using molecular dynamics to mimic quench from molten state at atomic scale. ► Characterisation of coordination environments of constituent species and analyses of glass topology and ring statistics. ► Prediction of LiAlF4 properties including glass transition temperature and thermal expansion coefficients. ► Identification of different regimes for lithium ion movement with both diffusive and sessile ions observed. ► Lithium diffusion mechanisms and calculated activation energies for diffusion are proposed.