In situ ⁷Li nuclear magnetic resonance study of the relaxation effect in practical lithium ion batteries

Lithium ion cells comprising actual components of positive electrodes (LiCoO₂, LiNi_xCo_yAl_z, and LiMn₂O₄) and negative electrodes (graphite and hard carbon) were assembled for in situ 7Li nuclear magnetic resonance (NMR) experiments. The ⁷Li NMR measurements of the cells revealed a ''relaxation effect'' after overcharging: a decrease of the signal assigned to Li metal deposited on the negative electrode surface by overcharging. The reduction of the Li metal signal was inversely proportional to the increase of the signal of lithium stored in carbon. Therefore, the effect was ascribed to absorption of deposited lithium into the carbon of negative electrodes. The effect, which occurred rapidly in a few hours, reached an equilibrium state at 8-15 h. The slight shift of deposited metal suggests that dendritic Li easily re-dissolved, although larger Li particles remained. A hard carbon electrode has a greater effect of Li metal relaxation than graphite electrodes do, which is explainable by the bufferable structure of the carbon. Results are expected to be important for the discussion of the state of lithium, and for safer battery design.