Molecular Dynamics Study of Lithium Diffusion in Lithium-Manganese Spinel Cathode Materials
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RANDALL T. CYGAN*, HENRY R. WESTRICH*, AND DANIEL H. DOUGHTY** Sandia National Laboratories, Geochemistry Department, Albuquerque, NM 87185-0750 Sandia National Laboratories, Lithium Battery Research and Development Department, Albuquerque, NM 87185-0613
ABSTRACT A series of molecular dynamics computer simulations of the self-diffusion of lithium in pure and several doped lithium-manganese spinel materials has been completed. The theoretical approach is part of an effort to understand the mechanisms and rates of lithium diffusion, and to evaluate the structural control of the cathode materials upon lithium intercalation (chargedischarge) process. The molecular dynamics approach employs a fully ionic forcefield that accounts for electrostatic, repulsive, and dispersion interactions among all ions. A reference unit cell comprised of 56 ions (Li 8Mn 3+8Mn 4+8032) is used to perform the simulations under constant volume and constant pressure constraints. All atomic positions are allowed to vary during the simulation. Simulations were completed for the undoped and doped LiMn 2O 4 at various levels of lithium content (based on the number of lithium ions per unit cell and manganese oxidation state). The molecular dynamics results indicate an activation energy of approximately 97 kJ/mole for self-diffusion of lithium in the undoped material. Lithium ion trajectories from the simulations provide diffusion coefficients that decrease by a factor of ten as the cathode accumulates lithium ions during discharge. Molecular dynamics results for the doped spinel suggest a decrease in the diffusion rate with increasing dopant ion. INTRODUCTION Molecular modeling and atomic-based energy calculations have recently been used to supplement the synthesis and testing of new oxide materials for lithium ion rechargeable batteries"12. The ability to derive a predictive model is critical to the development of new cathode materials and the improvement of battery performance. Of interest is the investigation of the effects of doping on the crystal chemistry, lattice constants, and electrochemical performance of the lithium manganese oxide spinels 3. The LiMn 20 4 spinel is one of the best oxide phases for a cathode material, having a voltage plateau of 4 V, a high specific capacity, high thermal stability, low cost, and no or little impact on the environment 4 . Lattice expansion and contraction during, respectively, lithiation and delithiation creates a buildup of stress in the cathode material, and can lead to significant degradation in the battery performance. By providing an atomistic description of lithium ion diffusion through the bulk LiMn2 O 4 crystal lattice, a molecular model will be able to evaluate possible diffusion mechanisms, and determine the relative diffusion rates of the lithium for dopant metal and dopant amount, and different levels of lithium intercalation. Our theoretical approach includes the use of an empirically-derived set of interatomic forcefield parameters to evaluate the stability and crystal structure of pure Li
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