Direct Observation of Li-Ions under Equilibrium and Non-Equilibrium Conditions By In Situ Neutron Depth Profiling

Neutron Depth Profiling allows to measure Li-ion concentration profiles perpendicular to the electrode plane. Incoming thermal neutrons are captured by ⁶Li (natural abundance 7.6%) producing charged ³H and ⁴He particles, the energy loss of which can be related to the depth of origin of the reaction, and hence of the lithium ions in the electrode. Using in-situ Neutron Depth Profiling (NDP) Li-concentration profiles were measured under various electrochemical conditions in LiFePO₄ electrodes. It is directly observed that at relatively high charge rate conditions (1C) lithium is depleted near the electrolyte side resulting in a lithium gradient over the electrode. During a subsequent rest period this relaxes towards a homogeneous depletion throughout the complete electrode removing the lithium gradient. At lower charge rates (0.1C, 0.2C and 0.5C) no gradient was observed indicating that the charging proceeds nearly in equilibrium. These direct observation illustrate that the charge transport in these electrodes is rate limiting and not the nucleation and diffusion in the LiFePO₄. By improving the ionic transport trough the electrodes by templating the concentration gradients were not observed at high charge rate conditions (1C), explaining the better performance of these templated electrodes. In addition aging tests were performed to locate the deactivated lithium in the electrodes. In summary the possibilities of NDP in Li-ion batteries are demonstrated under realistic electrochemical conditions.