Study on Cycle Life of Lithium-Ion Batteries Using in Situ 7Li Solid-State Nuclear Magnetic Resonance

The life of the lithium-ion batteries (LIBs) used as power sources for electrically powered vehicles, such as e-scooters, e-bikes, and electrically assisted wheel chairs, is critical for the stable operation of these products throughout their lifetime. Understanding the battery life may help in improving the performance of the vehicles powered by LIBs. However, examining the degradation mechanism without disassembling the cell is not easy. Thus, the cell life of the same cell cannot be tracked. Hence, in situ measurements are required.

We have developed an in situ ⁷Li solid-state nuclear magnetic resonance (NMR) measurement and studied the reaction inside the LIBs without decomposition [1,2]. We applied this technique to the analysis of the battery cycle life. The Li intercalation in carbon (graphite intercalation compound, GIC) was monitored every 100 cycles during cycle tests that were performed at 10 and 25 °C with 0.5 and 1 C discharge rates, respectively. The cell was composed of a LiCoO₂ positive electrode, a carbon negative electrode, and an organic electrolyte (ethylene carbonate (EC)/ethyl methyl carbonate (EMC) with 1 M LiPF₆). Fig. 1 shows the spectra measured at 100% state of charge (SOC), a fully charged mode at 0.2 C constant-current and constant-voltage (CCCV), every 100 cycles at 25 °C.

1. K. Gotoh, M. Izuka, J. Arai, Y. Okada, T. Sugiyama, K. Takeda, H. Ishida, Carbon, 79, 380 (2014).
2. J. Arai, Y. Okada, T. Sugiyama, K. Gotoh, M. Izuka, K. Takeda, ECS Trans. 62(1), 159 (2014).

Figure. 1. Spectra measured at 100% state of charge (SOC) (a) before the cycle test, (b) after 100 cycles, (c) after 200 cycles, and (d) after 300 cycles.