Lithium ion batteries have great potential to revolutionize the transportation sector as energy storage media for electric mobility. 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 presently exploited for automotive 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 loss. The dissolution of divalent manganese from the surface of the cathode into the electrolyte leads to a progressive decrease in the cathode material available for lithium intercalation. Deposition of dissolved manganese on the anode decreases the lithium content of the anode, further decreasing the overall cell capacity. Experimental observations indicate that the manganese content of the electrolyte increases as the cell ages, suggesting that cathode material loss is the primary reason for reduced cell life in a LiMn2O4 battery.
A key to improving the retention of manganese used as cathode material is to understand the reactions that occur at the cathode surface. Although Mn2+ has been experimentally detected in the electrolyte of LiMn2O4 cells, its speciation and reaction mechanisms are not yet well understood. Manganese dissolution is more pronounced in the presence of fluorinated electrolyte salts such as LiPF6, linking fluorine in the electrolyte as a putative trigger for manganese dissolution. Tunneling electron microscopy imaging has shown the presence of a layer of electrolyte decomposition products at the cathode surface, but no experimental composition analysis has been possible due to the thinness and delicate structure of this layer. Fluorinated electrolyte additives such as fluoroethylene carbonate (FEC) are used in commercial cells to enable formation of a more stable anode-electrolyte interface layer, thereby improving battery performance. The impact of this new source of fluorine on manganese dissolution and battery life has not yet been explored. To bridge this knowledge gap, molecular modeling and density functional theory methods were applied to investigate the reactions occurring at the LiMn2O4 cathode surface, the effects of fluorinated electrolyte additives on the cathode-electrolyte interface layer, and the mechanisms of manganese dissolution. Specifically, a reactive force field (ReaxFF) was developed to conduct molecular dynamics (MD) simulations of these electrochemical cells. Reactive force fields can simulate the formation and dissociation of chemical bonds, and thereby help to disclose the dynamic surface reaction chemistry of the cathode.
The reactive force field developed for manganese was used to investigate the formation of the cathodeelectrolyte interface layer in electrochemical cells with a LiMn2O4 cathode, ethylene carbonate / dimethyl carbonate electrolyte, LiPF6 salt, and 5% FEC additive. The MD simulations reveal that the electrolyte solvent molecules undergo oxidation by taking up oxygen atoms from the bulk cathode. The cathodeelectrolyte interface layer was found to contain aldehydes, polycarbonates and organic radicals. The oxidation reaction pathways for all the electrolyte solvent molecules reveal the formation of surface hydroxyl species. The removal of oxygen from the cathode lattice exposes manganese atoms to the bulk electrolyte and the hydroxyl molecules are very likely to result in acid formation on the cathode surface, these two processes together result in manganese dissolution from the cathode into the electrolyte. The chemical composition of the cathode surface interface layer with and without the electrolyte additive FEC, and the role of FEC in Mn dissolution were investigated. Simulation results generated by ReaxFF-based MD were found to be in agreement with the experimentally identified cathode-electrolyte interface compounds. Based on the Reax-FF MD simulations, the catalytic properties of manganese oxides lead to oxidative decomposition of electrolyte solvents resulting in consumption of cathode oxygen and dissolution of manganese atoms.