Probing Li-ion concentration in an operating lithium ion battery using in situ Raman spectroscopy

In situ Raman spectroscopy was employed to probe Li-ion concentration in liquid electrolytes at the cathode LiFePO4 particle surface in an optically transparent lithium ion battery. A Raman laser beam size of 2 μm was utilized so that Li-ion transport dynamics at the particle-level could be revealed. The variation of Li+ concentration in the LiPF6/ethylene carbonate (EC) + dimethyl carbonate (DMC) electrolyte was experimentally determined by analyzing Raman peaks corresponding to the C–O stretching vibration in EC and its relative intensity as to the Li+-solvent interaction in solvated products. The electrolyte concentration at the LiFePO4 particle surface was found to fluctuate during a battery charge-discharge cycle. Particularly, near the end of battery discharge, it reached to a minimum value (0.23 M) which was far less than its initial balanced value (1 M).