Second harmonic electron paramagnetic resonance spectroscopy and imaging reveal metallic lithium depositions in Li-ion batteries

We have investigated the metallic lithium particles nucleation following lithiation and delithiation steps of the graphite electrode using X-band electron paramagnetic resonance (EPR). Metallic lithium aggregates like dendrites and/or filaments which are formed during electrochemical cycling on the graphite anode are complex structures which may lead to internal short-circuit and safety issues. Understanding and following, in real conditions, this nucleation process is necessary to improve the development of Li-ion batteries. The complexity to detect metallic lithium structures inside Li-ion batteries depends on the number of EPR lines and their linewidth. The presence of lithiated graphite phases affects the detection of micrometric Li-metal elements. Herein, we report a new approach using cw-EPR spectroscopy and imaging combining the first and the second harmonic detection schemes to provide evidence of the metallic lithium aggregates nucleation in these negative electrodes. Although the first harmonic gives all the EPR signals present in the sample, it is found that the second harmonic EPR signal is mainly sensitive to metallic lithium depositions.