Lithium (Li)-ion cells have shown great promise as an enabler for clean, renewable energy storage. However, a key factor hindering their mass adoption in fielded renewable energy systems has been management of their functional performance for more effective battery utilization and control over their life. Over the course of charge and discharge cycles, the cell electrode materials undergo reversible material phase changes, known as intercalation stage transitions, which tend to drift or change as the cell ages or degrades. Monitoring these in real-time during fielded use would be very beneficial for effective cell state estimation in battery management systems (BMS). Presently these are only characterizable using special long-duration laboratory tests such as slow scan cyclic voltammetry. This paper focuses on the potential of hair-thin fiber-optic (FO) sensors to monitor intercalation stage transition points. Sharp features Were Observed in the signals from FO sensors bonded on cells at certain SOC values in a repeatable manner across different cycles at various C rates. These were demonstrated to correlate with intercalation stage transitions. This novel capability enabled by FO sensors can be useful both as a Li-ion cell design characterization tool and for run-time monitoring by BMS. (C) 2015 The Electrochemical Society. All rights reserved.