Degradation Analyses of Commercial Lithium-Ion Cells By C-Rate/Temperature Controlled Cycle Test

Introduction

There is an increasing demand for the performance and lifetime predictions of lithium-ion batteries under real operating conditions for their application into electric vehicles and energy storage systems. In order to establish a degradation model, the degradation factors, such as decomposition of electrolyte, formation of solid electrolyte interface (SEI), Li metal deposition, etc..., should be evaluated. In this paper, we conducted cycle tests using commercial 18650-type lithium ion cells at the conditions of three temperatures (0 °C, 25 °C, and 45 °C) and two current rates (1 C and 2 C). The electrodes and electrolytes of the degraded cells were also investigated by the several analytical methods.

Experimental

The active materials of cathode and anode are Li(Ni_1/3Mn_1/3Co_1/3)O₂ and graphite, respectively. The electrolyte is composed of EC, PC, EMC, and DMC, containing LiPF₆. The discharge capacities and internal resistances were periodically measured at 25 °C during the cycle tests. To understand the degradation mechanism, we disassembled the degraded cells and took out the electrodes and the electrolytic solution. The obtained electrolytic solutions were investigated by ¹H- and ¹⁹F-NMR, and GC-MS. The electrodes were investigated by ICP-MS, XRD, XPS, ⁷Li-NMR, and SEM. Moreover, the solid electrolyte interfaces (SEIs) of the electrodes were extracted by solvent and investigated by ¹H- and ¹⁹F-NMR.

Results and discussion

Fig. 1 shows the discharge capacity measured at 25 °C vs. cumulative discharge capacity plots during the cycle life tests. Surprisingly, the discharge capacities rapidly decreased at 0 °C conditions regardless of the cycle rates. As for 25 °C and 45 °C conditions, the discharge capacity decreased faster at 45 °C at 1 C rate, while discharge capacity decreased faster at 25 °C at 2 C rate. Fig. 2 shows the increase in the internal resistances measured at 25 °C, and again, the internal resistances increased rapidly at 0 °C, and the internal resistance at 25 °C increased faster than that at 45 °C at 2 C rate. At 1 C rate, the internal resistance at 25 °C increased faster than that at 45 °C before the drastic increase at around 1500 Ah.

The electrolyte analyses revealed that the decomposition of LiPF₆, which causes increase in internal resistance, occur faster at higher temperature conditions at both 1 C and 2 C cycle tests. As a consequence, the amount of inorganic metal fluoride SEI investigated by ¹⁹F-NMR was larger at higher temperature conditions. ¹H-NMR showed that the organic SEI formations also progressed at high temperature. These SEI formations consume active lithium for charge/discharge, and hence, the discharge capacities decrease. On the contrary, ⁷Li-NMR of anode showed that the metal Li tended to deposit at low temperature and high cycle rate. The surface SEM images drastically changed at high cycle rate, indicating the SEI formations of the anode proceed fast. In addition, XPS spectra showed that the component of the SEIs was different, that is, the ratio of carbonates was high at low temperature, and the ratio of phosphate was high at high temperature.

In summary, the observed cell degradations were caused by the several degradation factors which had different temperature/cycle rate dependences.