Ultrafast lithium migration in surface modified LiFePO4 by heterogeneous doping

The mechanism of the experimentally reported enhancement of lithium ion transport in LiFePO4 cathodes with glassy lithium diphosphate surface layers ultrafast (dis)charging of Li ion batteries is clarified by atomistic molecular dynamics simulations. A significant redistribution of Li+ from the phosphate glass surface layer into the subsurface LiFePO4 phase constitutes a rapid electrostatic storage component, and - more importantly - this Li+ redistribution constitutes a "heterogeneous doping" enhancing the defect concentrations on both sides of the interface. The resulting deviations from local electroneutrality qualitatively change the transport properties. For temperatures close to room temperature simulations yield an enhancement of ion mobilities in surface-modified LiFePO4 by up to three orders of magnitude via the mesoscopic multiphase effect. A layer-by-layer analysis of ion mobility in structurally relaxed hetero-structures indicates a continuous variation of the mobility as a function of the distance from the interface with the maximum mobility close to the interface. For nanoparticles of suitably chosen dimensions, Li+ diffusion remains enhanced compared to bulk values even at the center of the cathode material crystallites. Moreover, the role of Li-Fe(/)/Fe-li(center dot) antisite defects for the dimensionality of ion migration in bulk and nanostructured LiFePO4 is analyzed yielding criteria for a transition from one-dimensional to higher-dimensional long-range migration. This allows reconciling discrepancies between experimental single crystal studies and previous theoretical studies for the ordered LiFePO4 structure. (C) 2011 Elsevier Ltd. All rights reserved.