dV/dQ Curve Analyses of Degraded Lithium-Ion Batteries with Composite Cathode
Currently conducted lithium-ion batteries (LIBs) degradation analyses aim to improve the LIBs' life expectancy for electronic vehicles (EVs). As LIB for EVs are used in various environments, life tests at various conditions must be conducted. To clarify the relationship between test conditions and degradation mechanisms, as many LIBs as test conditions and cycle numbers must be analyzed. Hence, non-destructive analyses during life tests are needed for efficient and non-expensive degradation analyses. The dV/dQ curve analysis has been reported as a non-destructive degradation analytical method[1-2]. However, research on the relationship between test conditions (temperature, SOC range, and C-rate) and degradation factors through dV/dQ analysis has not been reported. In the present study, cycle-life tests on LIB with a composite cathode were conducted under several conditions, and the relationship between test conditions and degradation factors was quantitatively investigated using dV/dQ curve analyses.
A commercially available lithium-ion cell with a composite cathode (18650-type, 1.4 Ah) was used for the cycle tests. The cathode and anode consist of LiNi0.5Co0.2Mn0.3O2 + LiMn2O4 (75:25 wt.%) and graphite as active materials, respectively. Cycle tests were conducted at the following conditions: one charge/discharge C-rate (C/3), two SOC ranges (100–0%, 100–70%), and three temperatures (0°C, 25°C, 45°C). The low-current (C/20) charge/discharge test with the SOC range of 100–0% at 25°C was periodically executed during cycle tests for measuring the battery performance and calculating the dV/dQ curve from the discharge curve. The cathode (x LiNi0.5Co0.2Mn0.3O2・y LiMn2O4) and anode (graphite) dV/dQ curves were also obtained, which were calculated by the discharge curves of the half-cell against lithium metal electrode. The degraded cathode capacity, anode capacity, and the cathode/anode reaction region slip were diagnosed by curve fitting the cell's dV/dQ curve using the cathode and anode curves.
Results and discussion
Figure 1 shows the low-current discharge curves and dV/dQ curves of the cell, the cathode, and the anode of the initial cell (dotted line) and the degraded cell (solid line). Here, the degraded cell at test conditions of 45°C and 100–0% SOC range, after 480 cycles, is shown as an example. The capacity retention of the cell was 82.6%, and the anode reaction region's shift and the cathode capacity's decrease were observed. Figure 2 shows the cathode degradation and the cathode/anode reaction region slip during the tests at SOC range of 100–0% and 45°C. Both result in a decrease of the cell's capacity, and the cathode/anode reaction region slip progress faster.
The conducted dV/dQ analyses for all the test conditions indicate that the influence of the various degradation factors (cathode degradation, anode degradation, and cathode/anode reaction region slip) differs according to the cycle test conditions. When the SOC range is wide, the cathode degradation proceeds faster. This may be due to the cathode material's (especially LiNi0.5Co0.2Mn0.3O2) degradation, which is caused by the electronic isolation and/or inactivation by expansion and contraction in the charge/discharge tests. In contrast, when the temperature is high, the cathode/anode reaction region slip accelerates the cell's capacity decrease, probably because the reaction region slip results from the rechargeable lithium-ion consumption through electrolyte decomposing (as a side reaction), which is accelerated by temperature.
In conclusion, dV/dQ curve analyses were applied on LIBs with a composite cathode. The results indicate that the cathode degradation and the cathode/anode reaction region slip cause the cell's capacity to decrease, and their effect differs according to the test conditions.
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