The capacity fading of lithium ion batteries (LIBs) is investigated, using in situ Li-7 solid-state nuclear magnetic resonance (NMR). LIB cells consisting of graphite or hard carbon and LiCoO2 are used and cycled under two different temperatures (10 and 25 degrees C) and charge current rates (0.5 and 1 C). The cell capacity and the amount of Li stored in carbon are measured with a battery cycler and in situ 7Li solid-state NMR measurements before the beginning of the test and after every 100 test cycles. The in situ 7Li solid state NMRs provide sufficient information throughout the cycle tests to characterize all test conditions. The cell capacities are analyzed in terms of the 7Li NMR peak intensity, attributed to Li stored in carbon. This intensity shows affine proportionality with the cell capacity for every evaluation, with an additive constant that decreases with the increase in cell capacity fading. This may be related to the loss of Li storage caused by voltage "slippage." (C) The Author(s) 2016. Published by ECS.