Introduction With the recent increase in energy demand, lithium-ion batteries (LIBs), which have high lifetime characteristics and energy density, are being considered for batteries used to stabilize power supply and to store renewable energy. However, the detailed degradation mechanism over a wide range has not yet been clarified. Previous studies have reported that float charging, which assumes the use of LIBs as backup power sources, and the combination of float charging and occasional discharge at room temperature conditions cause significant degradation.1 On the other hand, as suggested by the fact that battery acceleration tests are generally conducted at high temperatures, it is important to understand the degradation conditions at high temperatures. In fact, heat is generated by charging and discharging when large storage batteries are used. Considering the above, the purpose of this study is to clarify the temperature dependence in degradation by conducting degradation tests combining float charge and occasional discharge at 25°C and 40°C in comparison. Experimental Commercially available 18650 cylindrical LIBs (Panasonic, NCR18650R, nominal capacity: 3350 mAh), presumably from the same lot, were used for the degradation test. The uniform performance was validated with some cycling tests. Degradation tests comprise three key parameters: (1) charging and discharging patterns, (2) operating voltage range, and (3) operating temperature. The detailed conditions are shown in Table 1. A total of 12 patterns of degradation tests are conducted under combinations of these conditions. Analysis of the degradation status is regularly performed by differential voltage analysis of discharge curves, electrochemical impedance measurement, and operando neutron diffraction measurement as non-disassembly analysis.Operando neutron diffraction measurements were used to measure changes in the crystal structure of the positive electrode. Diffraction measurements were performed at room temperature using a time-of-flight powder diffractometer at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Research Complex2. The test cell was first charged at 0.5C (1675 mAh) with a cutoff potential of 4.2 V, followed by constant-voltage charging at a cutoff current of 0.02C (67 mAh), to achieve a fully charged state, and then the diffraction profile in the open circuit state was measured after 1 h rest. Next, diffraction measurements were performed while discharging at a constant current of 0.1 C (335 mAh) up to 2.5 V. One diffraction profile was obtained from the integrated data of signals obtained during 10 minutes. Finally, the diffraction measurement was completed with full discharge. The Rietvelt analysis was performed using the diffraction profiles obtained in these measurements to refine crystal parameters such as lattice constants and mass ratios, which indicate the existence ratio of each constituent material. Results and discussion After 100 days or 100 cycles, the most degraded condition is the mode that combines float charging and once-a-day discharge, operating in a high SOC region and at high temperatures. The differential voltage analysis of discharge curves showed that the loss of lithium inventory (LLI) was rarely observed for the mere float charge whereas LLI was clearly found when the combination of float charging and occasional discharge was employed. It was also confirmed that LLI occurred more when the degradation was operated under high temperature conditions. On the other hand, electrochemical impedance measurements showed that the internal resistance of the positive electrode was significantly increased than that of the negative electrode, suggesting the resistance increase in the positive electrode (RIPE) was caused by the species formed at the negative electrodes re-oxidized at the positive electrode.The operando neutron diffraction profiles of the fresh cells were measured and determined to be layered positive electrode material for the positive electrode and graphite for the negative electrode. The 003 diffraction profiles of the layered positive electrode material in the fresh cell or degraded cell are shown in Fig. 1. Comparing the fresh cell and degraded cell, a broadening of the profile is observed in the early stage of discharging in the latter, which suggests the RIPE causing the charge-discharge reaction distribution. These results indicate that the combination of float charging and discharging under high temperature conditions not only causes LLI, but also significantly causes the RIPE mode, resulting in capacity degradation.
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Jan 13, 2025
Zi-Ying Yang + 4
Just Published
Open Access