An SEI evaluation method for designing the optimal lithium-ion battery formation protocol. In lithium-ion cells, the electrochemical stability of the solid electrolyte interphase (SEI) formed during the forming process directly affects the long-term performance of the cell. To derive such protocols, the electrochemical stability of SEI under each formation protocol should be compared.
The passivation ability was evaluated by the capacitance ratio during the double potential step and the interfacial kinetics is evaluated by interpreting the current change with the inverse Cottrell plot. In the form of an inverse Cottrell plot, the contributions of interfacial resistance and ohmic drop to the chronoamperometric response can be separated. As a result, it is proved that the designed method is one of the effective methods for selecting the formation protocol as the optimal protocol derived from the double potential step shows the highest capacity retention and scale ability.
The change in current (a) charge (b) discharge, and (c) capacity and (d) the capacity ratio, DOP value of each protocol during double potential step (1.0V – 0.6V). Each resistance of (d) before and (e) after double potential step. a) Full cell voltage corresponding to the FEC response potential. The capacity retention of 1C 100 cycle test and (b) rate capability test after each formation cycle. a) The FEC-derived SEI (b) The increase in diffusion carrier concentration due to the formation of LiF interface.
XPS surface composition (atomic percentage) of the disassembled graphite anode after forming and 100 cycles of FEC@1C and [email protected].
Introduction
The importance of the formation process
The necessity of the SEI evaluation method
Experimental
Electrode preparation and cell fabrication
The anode area is about 1.2 times that of the cathode to prevent the growth of lithium metal at the edge of the anode.
Electrochemical measurements and post-mortem characterization
Results and discussion
Methodology
- The principle of method: Double potential step
- Introduction of verified protocols
- Introduction of the set protocols
Therefore, measuring the amount of additional electrolyte decomposition during a double potential step can be an indicator of the passivation capacity of previously formed SEI. In other words, the high DOP value means that the passivation ability of the SEI formed in the formation cycle is high. Figure 7 shows the change of DOP value when the double potential step is repeated eleven times with the fresh Li/Graphite half-cell.
Through this experiment, it was verified that the degree of the additional SEI reaction can be compared quantitatively with the DOP value obtained by the evaluation method, double potential step. In this case, the outline of the graph in the inverse Cottrell plot is shown in the form of a continuous horizontal line (dotted orange) shown in Fig. The trend of the DOP value obtained by the evaluation method is related to the cycle retention results.
The result of the cycle test and rate capability test according to each protocol is presented in the paper as shown in Fig 11. This trend of the slope of each protocol is consistent with the rate capability test result shown in Fig 11b. Thus, it was verified that the trend of cycle retention and rate capability in 10 hours can be predicted by the double potential step method which can compare the passivation capability and interfacial kinetics of the formed SEI.
Full-cell performance testing was conducted using pouch cells assembled in the laboratory, and the results of the test were proven to be consistent with the paper. Therefore, this proved the effectiveness of the evaluation method comparing each protocol through the DOP value. And there was no significant difference in the capacity and efficiency of the half-cell formation cycle.
Passivation ability with DOP value – After each forming cycle, the charge/discharge capacity of the double potential step is shown in Fig 16. The passivation ability of the formed SEI was the highest in the protocol (FEC@1C) with C- at the highest rate at the FEC reaction potential, 1.18 V. According to the result of the double potential step, it was confirmed that the interfacial kinetics was the highest at FEC@1C.
In other words, this analysis disproved that the passivation capability of the FEC@1C protocol was greater. As a result of EIS and XPS characterization, it was verified that it is effective to quantitatively compare the SEI properties of each condition with the DOP value and the linear slope of the inverse Cottrell plot through the double potential step. This resistance can be obtained from the slope of an inverse Cottrell analysis using the double potential step applied and the change in response current.
During a double potential step, the slope results from the contribution of the SEI resistance, depending on the interfacial kinetics of the preformed SEI.
High-precision coulometric studies of the influence of temperature and time on SEI formation in Li-ion cells. Effect of slow interfacial kinetics on the chronoamperometric response of lithiated composite graphite electrodes and on the calculation of the chemical diffusion coefficient of lithium ions in graphite. Discussion of the potential step method for the determination of diffusion coefficients of guest species in host materials: Part I.
Synergistic effects of inorganic components in the interphase of solid electrolytes on the high cycle efficiency of lithium ion batteries. First of all, I would like to thank Professor Kyeong-Min Jeong for mentoring me over the past two years. Thanks to my advisor, I was able to grow little by little as a battery engineer who wants to study and not just graduate.
From now on, I will do my best to become a disciple who achieves what is essential to society. Also, I want to thank the members of the lab who helped me a lot. In particular, I want to apologize and thank you for taking so long.
Thanks to UNIST, I was able to find my purpose in life by growing up for 8 years in a good environment with wonderful people. Finally, I would like to dedicate my graduation to my parents, whom I respect and love the most in the world.
Acknowledgements
