In situ solid state ⁷Li NMR observations of lithium metal deposition during overcharge in Lithium Ion Batteries

Lithium metal deposition during overcharge in practical lithium ion cells composed of a lithium metal oxide (LiCoO₂) positive electrode coated on Al foil, carbon (synthesized graphite and hard carbon) negative electrodes coated on Cu foil, polypropylene separator, and liquid electrolyte was observed using in situ solid state ⁷Li nuclear magnetic resonance (NMR) measurements with an original probe featuring a flattened solenoid coil. Li insertion and extraction in carbon electrodes were monitored during charge and discharge and the intensities of certain peaks were found to be proportional to the cell capacity change. The deposition of metallic Li commenced after the cell voltage exceeded the nominal value and almost saturated after 160% of charge at a low current rate. The measurements showed that the deposition of metallic Li was much easier on graphite compared to hard carbon. The metallic Li deposited on hard carbon was almost completely discharged, whereas that on graphite remained after discharging to 2.5 V.