During operation of a Li-ion cell, lithium diffuses in and out of the solid phase anode and cathode. Measuring the lithium concentration within the electrodes of the cell over its operational life could provide key information for understanding the loss of active lithium and the associated aging mechanisms. Concentration profiles of lithium in a cell are difficult to measure with traditional spectroscopic techniques. Here, neutron depth profiling was used to measure lithium near-surface concentration profiles along the anode and cathode strips in off-the-shelf cylindrical Li-ion batteries as a function of the aging process. A buildup of the surface concentration of lithium was found in graphite anode as well as a general decrease in the intercalation efficiency in lithium iron phosphate cathode material with aging. (C) 2011 Elsevier Ltd. All rights reserved.