III. Results and discussion
3.1 Methodology
3.3.1 Introduction of the set protocols
Fluorethylene carbonate (FEC), which increases the composition of inorganic species in SEI, is the most widely used electrolyte additive. In this study, each step charge half-cell protocol (1st + 2nd) was set as shown in Table 5 by varying the electrochemical conditions to reach the FEC reaction potential of 1.18V. To check the effect of the current and time at the FEC reaction potential on the SEI, the C- rate and cut-off condition of the 1st step were set as variables, respectively. After 1st step, both the 2nd step and the discharge condition are the same.
Table 5. Step charge protocol set based on the FEC reaction potential and the capacity of each formation cycle.
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FEC@1C is a protocol applied with 1C up to 1.18V and [email protected] is a protocol applied with 0.05C up to 1.18V. These two types of protocols differ not only in the current but also in the time required for the 1st step. [email protected] takes longer than FEC@1C in the 1st step. Therefore, in addition to the current variable, a protocol that can check the effect of time in 1st step was needed. The protocol set to check the effect of time is FEC@1C_time. In FEC@1C_time, the current of the 1st step is 1C, but by different cut-off conditions, the time to stay in the 1st step was set longer than that of [email protected].
The formation capacity and required time of the set protocol are shown in Table 5 and Fig.15. It can be assumed that there is little difference in SEI thickness because the total formation time is similar.
And there was no significant difference in the capacity and efficiency of the half-cell formation cycle.
Thus, it is difficult to identify trends according to each protocol as a result of the 1st cycle.
Fig. 15. The charge (a) and discharge (b) voltage profile of FEC@1C, [email protected], and FEC@1C_time
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3.3.2 Derivation of optimal protocol through double potential step
Passivation ability with DOP value – After each formation cycle, the charge/discharge capacity of the double potential step is shown in Fig 16. The constant voltage discharge capacity is almost similar, but the difference in the capacity ratio (DOP) occurs due to the difference in charge capacity.
Since the FEC@1C protocol has the lowest charge capacity, the SEI reaction occurred the least during the charge step, and this resulted in the highest DOP value. On the other hand, [email protected], which had the highest charge capacity, showed the lowest DOP value. In the case of FEC@1C_time, which increased the time compared to FEC@1C, the constant voltage charge capacity increased further, and the DOP value decreased. Through this, it was confirmed that the current variable has a greater effect on the DOP than the time of the 1st step and the DOP value did not increase due to the longer 1st step time. The passivation ability of the formed SEI was the highest in the protocol (FEC@1C) with the highest C-rate at the FEC reaction potential, 1.18V.
Fig. 16. 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).
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Interfacial kinetics with inverse Cottrell plot – Fig. 17 shows the change in the current during the constant voltage discharge step of each protocol as an inverse Cottrell plot. As mentioned in the previous section, the larger the linear slope of the inverse Cottrell plot, the greater the influence of the interfacial resistance and ohmic drop. From the results of EIS measurement before/after the double potential step, the ohmic drop was all the same. Comparing each linear slope, the linear slope of the FEC@1C was the lowest and the slope of [email protected] was the highest. In addition, since the
intercept of each straight line is the same, it can be seen that the reason for the different slope is due to the kinetic parameter (Λ). According to the result of the double potential step, it was confirmed that interfacial kinetics was highest in FEC@1C.
Consequently, as a result of evaluating the passivation ability and interfacial kinetics of each protocol by applying the double potential step, the optimal formation was FEC@1C.
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Fig. 17. The linear slopes of inverse Cottrell plot of each protocol, 1.0V-0.6V
Table 6. The DOP values and slopes of the inverse Cottrell plot for each protocol
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3.3.3 Validation of optimal protocol (1): EIS measurement
Electrochemcial impedancec spectroscopy (EIS) measurement – As shown in Table 7, EIS
measurement of Li/graphite half-cell at 1.0V is performed before/after the double potential step. The semi-circle of the general Nyquist plot includes charge transfer resistance, which is the case when the charge transfer resistance is small (medium SOC or higher). But, at 1.0V (low SOC), since the
reactant concentration participating in the electron transfer is small (low exchange current), the charge transfer resistance is very large and cannot be included in the semi-circle of the Nyquist plot.
Therefore, it is possible to check only the change in film resistance through the change in the semi- circle before/after the double potential step.Fig. 18 is the result of measuring ohmic/contact resistance and film resistance before/after the test for each formation protocol. In the case of ohmic/contact resistance, there was not only a little difference before the test, but also the rate of increase after the test was similar. However, in the case of film resistance, the difference between protocols was clearly observed. In Table. 7, the increase rate of film resistance from [email protected] was the highest after the test, followed by FEC@1C_time and FEC@1C. The increase rate in film
resistance depends on the passivation ability as it results from SEI growth during potential step. As a result of EIS measurement, the SEI layer from FEC@1C protocol with the highest passivation ability is most robust and it is consistent with the results of the DOP value.
Fig. 18. Nyquist of (a) before and (b) after double potential step. (c) Equivalent circuit model for Z-fit. Each resistance of (d) before and (e) after double potential step.
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Also, the film resistance before/after the double potential step is lowest in FEC@1C, which was evaluated as having the highest interfacial kinetics. Interfacial kinetics represents the barrier that the lithium-ion needs to overcome to cross the interface between the electrolyte and the electrode including the de-solvation step, transport in the SEI step, and electron transfer at the anode surface.
The ion transport in SEI increases at low film resistance, resulting in decreased interfacial kinetics and it is consistent with the results of the linear slope of inverse Cottrell plot.
Thus, through the EIS measurement before/after the potential step, it was confirmed that it is valid to compare the passivation ability and interfacial kinetics of each protocol with the devised evaluation method, double potential step.
Table 7. Each resistance obtained by fitting to an equivalent circuit model
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3.3.4 Validation of optimal protocol (2): cycle test & rate capability test
In this section, full cell tests were conducted to verify that the previously derived optimal condition represents the highest full cell performance.
Full cell formation protocol design - By applying the formation protocol of Li/Graphite half-cell equally to the formation of the full cell (Graphite/NCM622), the validity of derived optimal condition through the double potential step was verified. The full cell protocols suitable for half-cell were set as shown in Table 8 with 3-electrode cell test. The FEC reaction potential (1.18V) was set to 2.7V based on the full cell voltage (Fig. 19a), and the step charge protocol was designed with C-rate and cut-off conditions as before.
Fig. 19. (a) Full cell voltage corresponding to the FEC reaction potential. (b) Charge and (c) discharge voltage profile of each full cell formation.
Table 8. Full cell protocols set based on the half-cell formation protocols
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Full cell performance test – After each formation cycle, 1C 100 cycle test and rate capability test were conducted as shown in Fig 20. In the case of the capacity retention for 100 cycles, it was confirmed that the retention was the highest in FEC@1C, which had the highest DOP value in the evaluation test of Li/Graphite cell. Also, in the case of the discharge rate capability test, it was confirmed that the discharge capacity was highest at the high C-rate in FEC@1C. The previous evaluation of interfacial kinetics through the inverse Cottrell plot was also the highest in FEC@1C.
Therefore, the effectiveness of the devised evaluation method in this study was verified by confirming that the derived optimal protocol through double potential step has the greatest full cell performance.
Fig. 20. (a) The capacity retention of 1C 100 cycle test and (b) rate capability test after each formation cycle.
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3.3.5 Post-mortem analysis: X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) analysis - The electrochemical properties of the SEI layer are determined by the SEI chemistry. Through XPS analysis, the difference in passivation ability and interfacial kinetics can be interpreted in terms of SEI chemistry. After each formation cycle, the full cell was disassembled and XPS analysis of the graphite surface was performed. The analysis was conducted mainly through the F1s peak including the inorganic species like LiF (686 eV) and O1s peak including the organic species like alkyl carbonate (532 eV). The inorganic LiF, which has relatively low electronic conductivity in SEI, plays a role in inhibiting additional electrolyte decomposition by preventing the movement of electrons toward the electrolyte. In addition, LiF in SEI facilitates lithium-ion transport by increasing the diffusion carrier concentration (Fig. 21b).19 For this reason, related papers have shown that it is advantageous for performance to form FEC-derived SEI (Fig. 21a) with a high LiF ratio by adding FEC.20 Also, since FEC is preferentially reduced than solvents such as EC in FEC-derived SEI, there are relatively few organic species, so the related O1s peak appears at lower intensity in XPS.21 Therefore, the difference in passivation ability and
interfacial kinetics of each protocol is explained by the difference in F1s and O1s peak.
Fig. 21. (a) The FEC-derived SEI (b) The increase of diffusion carrier concentration due to the formation of LiF interface.
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As shown in Fig. 22, the LiF peak (686 eV) in the FEC@1C tends to be relatively higher than the [email protected] after the formation cycle. Comparing the surface atomic concentration in Table 9, the ratio of fluorine (F1s) is high, and the ratio of oxygen (O1s) is low in FEC@1C. Since this trend is identical in the depth profile, it can be confirmed that FEC@1C is relatively closer to the FEC-derived SEI. Thus, the result of XPS analysis is consistent with the trend of DOP and linear slope of inverse Cottrell plot. However, in the same 1C protocol (FEC@1C/FEC@1C_time), the XPS results were almost similar, making it difficult to compare. That is, there is a limitation that comparison of protocols through post-mortem results such as XPS analysis is effective only when there is an identifiable SEI composition difference.
From the results of XPS analysis after 100 cycles, the difference in the oxygen concentration in the surface region increased further. So, it can be estimated that additional electrolyte decomposition occurred relatively more at [email protected] during the cycle test. In other words, this analysis disproved that the passivation ability of the FEC@1C protocol was higher.
As a result of the characterization with EIS and XPS, it was verified that it is effective to compare quantitatively the SEI properties of each condition with DOP value and linear slope of inverse Cottrell plot through the double potential step.
Table 9. XPS surface composition (atomic percentage) of the disassembled graphite anode after formation and 100 cycles of FEC@1C and [email protected]
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Fig. 22. XPS spectra (F 1s, O 1s) for the disassembled graphite anode after formation cycle (a, b) and 1C 100 cycles (c, d) of FEC@1C and [email protected]
Fig. 23. XPS depth profiles for the disassembled graphite anode after formation cycle (a, b) and 1C 100 cycle (c, d) of FEC@1C and [email protected]
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IV. Conclusion
In this work, we present a novel method to evaluate the characteristics of SEI after formation cycles using double potential step. After the formation cycles, when the Li/Graphite half-cell was applied with the double potential step, the current change during applying the potential varied depending on the formation cycle conditions. Differences in current change according to formation cycle conditions may be due to the differences in SEIs formed during formation cycles. It could be demonstrated that the properties of SEI can be evaluated by interpreting the current change during potential steps. The characteristics of SEI can be divided into passivation ability and interfacial kinetics. First, the passivation ability is evaluated as degree of passivation (DOP), which is the constant voltage charge/discharge capacity ratio, and the DOP changes as additional irreversible SEIs are formed.
Second, the interfacial kinetics was measured as the resistance of SEI. This resistance can be obtained from the slope of the inverse Cottrell analysis using the applied double potential step and response current change. During double potential step, the slope results from the contribution of the SEI resistance depending on the interfacial kinetics of the pre-formed SEI. The predicted performance from the new method and the results of the full cell test were well-matched. Therefore, this means that each formation protocol can be evaluated in less than 10 hours through the double potential step instead of time-consuming full cell tests.
Additionally, this evaluation method can be used not only for formation protocol evaluation but also for cell operation. The double potential step allows you to determine how far the SEI degradation is progressing before and after cell operation. However, in order to apply to cell operation, it is thought that appropriate potential steps for the full cell need to be identified.
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