In situ solid state ⁷Li NMR observation of lithium metal deposition during overcharge in lithium ion battery

Lithium metal deposition during overcharge in a full cell composed of lithium metal oxide (LiCoO₂) positive electrodes and carbon negative electrodes (a synthesized graphite, a natural graphite and two types of hard carbon) was observed by in situ ⁷Li solid state NMR (nuclear magnetic resonance). Peak intensities corresponding to the GIC (Graphite Intercalation Compound, charged state) and dendritic Li (deposited metallic Li) were monitored at each 20% state of charge (SOC) during discharge (and charge) and each 17 minutes (or 1000 scans) after the cell capacity reached the expected overcharge. The GIC peak increased with charge and decreased with discharge during the normal voltage of operation. The Li metal deposition occurred after the cell was charged above its normal operational voltage (4.2 V) though intercalation into the carbon still simultaneously occured, suggesting that metallic Li can be formed when the negative electrode reaches a voltage below 0 V vs. Li. We also found that the dendritic Li is not inert but active to be oxidized again and intercalated into the carbon within a few hours. The dendritic Li formed on hard carbon was completely discharged, while that deposited on graphite was not, indicating that the electrochemical property of the dendritic Li varies with the nature of the negative electrode.