(775b) A Reactive Force Field for Manganese-Rich Cathodes in Electrochemical Systems

Lithium ion batteries are now the most widely used rechargeable systems.  Owing to their high voltage and high charge density, they have replaced conventional alkaline and nickel based batteries for portable devices.  They are now also being incorporated into high power applications like hybrid electric vehicles (HEV).  Among the various lithium metal oxides that have been used, or proposed for use, as cathode active materials, lithium manganese oxide (LiMn₂O₄) is the primary cathode material being exploited for HEV applications, on account of its low cost, low toxicity, and simple preparation process.  There are, however, well documented problems with LiMn₂O₄, the most critical of which is capacity fade due to active material dissolution. The LiMn₂O₄ spinel has an equal proportion of manganese ions in two oxidation states, Mn³⁺ and Mn⁴⁺, yielding a mean manganese oxidation state of +3.5.  Conventional wisdom holds that trivalent manganese ions are unstable to acid attack (e.g. from hydrofluoric acid formed by electrolyte reactions), and are lost from the cathode surface into the electrolyte following charge disproportionation through the Hunter reaction 2 Mn³⁺ <--> Mn⁴⁺_solid + Mn²⁺_solution.  Dissolution of divalent manganese into the electrolyte reduces the effective amount of functional cathode material and also decreases the Li content of the anode, and hence the overall cell capacity, on account of manganese deposition at the anode-electrolyte interface.  Therefore, the key to improving the performance of manganese-rich lithium oxide cathode materials is to better understand the Mn dissolution reactions occurring at cathode-electrolyte interface, so as to devise strategies (e.g. coatings, dopants, electrolyte compositions) that curb dissolution and increase the spinel electrode stability.

Force fields based on bond order and bond energy have been developed to simulate bond formation and dissociation, and hence accurately describe reactive systems.  The Reactive Force Field (ReaxFF) is one such force field developed to bridge the gap between quantum chemical and empirical force field based computations.   ReaxFF has been developed for a wide range of systems including hydrocarbon oxidation, pure metallic systems, catalytic oxidation of metal oxides, and evolution of the anodic solid-electrolyte interface (SEI) in Li-ion batteries.   In this paper, we report on our progress to extend ReaxFF so as to incorporate mangenese and fluorine for intended applications in ReaxFF molecular dynamics (MD) simulations of lithium ion battery cells.  To capture the chemistries of manganese and fluorine, a DFT training set has been developed consisting of approximately one thousand structures and computed energetics related to bond dissociation, angle and dihedral distortions, and reactions with the other elements in the lithium cell (C, H, O, Li) whose force fields are known and transferable to the present ReaxFF application.  Preliminary results are presented for ReaxFF MD simulations of the cathode and the cathode SEI layer spatial composition.