As mentioned in
Figure 1, the three factors of performance (capacity, DCIR, and AC-impedance) were measured at 30 repetitions of each pattern to diagnose and analyze the battery deterioration.
Figure 4,
Figure 5 and
Figure 6 show the results of the battery performance test under the 30 pattern repetitions for each.
3.2.1. Battery Performance Test: Capacity
Figure 4 shows the result of capacity measurements after 330 repetitions of V0G, V1G, and V2G patterns. The decrease in battery capacity with the increase in pattern repetition can be seen. In (a), the battery capacity following V0G pattern repetitions is shown to decrease from the initial 54.97 Ah (State of Health (SOH) 100%) to 52.44 Ah (SOH 95.40%). Similar results are visible for V1G and V2G. In (b), the battery capacity following V1G pattern repetitions is shown to decrease from the initial 54.35 Ah (SOH 100%) to 51.55 Ah (SOH 94.84%). The battery capacity following V2G pattern repetitions is also shown to decrease from the initial 54.79 Ah (SOH 100%) to 51.54 Ah (SOH 94.07%).
Figure 4d shows the comparison of capacity change following the V0G, V1G, and V2G pattern repetitions.
SOH after 330 pattern repetitions was 95.40% for V0G, 94.84% for V1G, and 94.07% for V2G, indicating the capacity fades quickly in the order of V2G, V1G and V0G. This may be due to two reasons. The first is the influence of the calendar life. The operation time of a single cycle for each pattern was 18 h for V0G, 21 h for V1G, and 24 h for V2G, with 3 h difference per cycle and approximately 90 h difference for 30 pattern repetitions, so that the influence of calendar life led to the variation in battery deterioration. Another reason is the difference in the quantity of charge/discharge. V2G has a higher level of charge-discharge than V0G or V1G, which could lead to the variation in battery deterioration. For future causal analysis, the completion time of each pattern is to be set identical to prevent the influence of calendar life in testing the battery deterioration according to pattern.
3.2.2. Battery Performance Test: DCIR
DCIR is calculated based on Ohm’s law;
R = Δ
V/
Iavg. The ohmic resistance for the whole cell of DCIR,
R1 (
Rs in AC-impedance), is calculated based on the difference between the voltage measured immediately after the application of discharge pulse (
V1) and the voltage without load (
V0) (1). The charge transfer resistance,
R2 (
Rct in AC-impedance), is calculated based on the difference between the voltage measured after the application of discharge pulse for
n sec (
Vn) and the voltage measured immediately after the application of discharge pulse (
V1) (2).
As a result, the discharge pulse current is identical for
R1 and
R2 so that, after the application of voltage, the voltage (
V1,
Vn) becomes the key determinant. In addition, the voltage after application is determined based on the time of data recording, and the result of recording with 1 sec interval is shown in
Figure 7b.
Figure 7 shows the result of comparing the IR between AC-impedance and DCIR of a fresh cell for the V0G pattern. The DCIR immediately after the application of current was 1.049 mΩ,
R1: 1.049 mΩ, while the AC-impedance was 0.981 mΩ (
Rs: 0.699 mΩ,
Rct: 0.282 mΩ) (
Figure 6a). As shown, the
R1 of DCIR is greater than the
Rs +
Rct of AC-impedance. This is because, immediately after the current is applied, the resistance corresponding to
Rs as well as
Rct are measured together for the DCIR. For this, the DCIR result was analyzed based on the data obtained after 1 sec. In future,
R1 and
R2 values are to be extracted based on the data recording of 1 ms or 10 ms interval to analyze the relationship between the DCIR and AC-impedance.
Figure 5 shows the result of measuring the IR through DCIR after 330 repetitions of V0G, V1G, and V2G patterns. The IR was measured after 30 pattern repetitions at 50% SOC. Next, the IR was calculated based on the voltage measured after applying the current of 1.0 C (55.6 A). Notably, the IR in this study was calculated using the voltage measured after 1 sec application of the current (
Figure 5). The initial IR of the battery used in the operation of V0G, V1G, and V2G patterns was 1.049 mΩ, 1.043 mΩ, and 1.079 mΩ, respectively. Up to 60 pattern repetitions, the IR decreased (V0G: 0.935 mΩ, V1G: 0.941 mΩ, V2G: 0.942 mΩ). Afterwards, the battery deterioration with increased pattern repetition led to a rise in IR to 1.034 mΩ, 1.078 mΩ, and 1.059 mΩ for V0G, V1G, and V2G, respectively, after 330 repetitions. As a result, a solid electrolyte interface (SEI) layer is formed through the initial charge-discharge, where the SEI layer stabilizes while preventing the continuous degradation of the electrolyte on the anode surface [
11,
12]. In addition, the gas produced through battery charge-discharge is thought to have increased the internal pressure to improve the contact performance and ultimately reduce the IR [
13,
14]. After 60 pattern repetitions, the IR increased owing various causes with the progression of charge-discharge, such as the growth of the SEI layer, consumption of the electrolyte, and the damage on the SEI layer [
11,
12]. Similar results are observed in the AC-impedance analysis (
Figure 6).
3.2.3. Battery Performance Test: AC-Impedance
As mentioned in
Figure 1, the electrochemical impedance spectroscopy (EIS) was performed after each of the 30 pattern repetitions. The AC-impedance was measured as the applied frequency was reduced from high to low, and as a result, the ohmic resistance,
Rs, was measured at high frequency, and the charge transfer resistance,
Rct, was measured at medium frequency.
To extract the factors of battery characteristics, the EIS was performed at 50% SOC, and the potentiostatic method was used to apply the AC waveform of 10 mV voltage amplitude and for the measurements from 10 kHz up to 100 mHz.
Figure 6 shows the result of IR measurements for AC-impedance after 330 repetitions of the V0G, V1G, and V2G patterns. As in the case of DCIR, the IR decreased for up to 60 repetitions for each pattern. Afterwards, the IR increased as the number of pattern repetition increased.
Such trend was highly similar to the trend exhibited by DCIR. While the absolute values may vary, the trend of IR decrease for up to 60 pattern repetitions, followed by a gradual increase in IR with an increase in pattern repetitions, appears almost identical. Thus, the DCIR is considered to be as useful in analyzing and selecting degraded batteries.
In addition, the
Rs and
Rct values of AC-impedance were comparatively analyzed.
Figure 6e is the result of comparing the ohmic resistance,
Rs, per pattern.
Figure 6f is the result of comparing the charge transfer resistance,
Rct, per pattern. An approximately 0.1 mΩ decrease can be seen, from the initial values; 0.282 mΩ for V0G, 0.216 mΩ for V1G and 0.373 mΩ for V2G to 0.185 mΩ for V0G, 0.136 mΩ for V1G and 0.238 mΩ for V2G after 60 pattern repetitions. In contrast, for
Rs, the initial values decreased from 0.699 mΩ for V0G, 0.758 mΩ for V1G and 0.641 mΩ for V2G to 0.691 mΩ for V0G, 0.736 mΩ for V1G and 0.635 mΩ for V2G after 60 pattern repetitions, by approximately 0.015 mΩ. Thus, up to 60 pattern repetitions, the value of
Rct plays a key role in the changes in IR. This is because the IR for
Rct falls as the SEI layer is formed upon the initial charge-discharge. [
11,
12,
15,
16]. It is also because the gas formed during charge-discharge increases the internal pressure to improve the contact between the electrode and binder [
17]. Moreover, the decrease in
Rs was also by approximately 0.015 mΩ, owing to the increase in ionic conductivity after the SEI layer formation [
13,
14].
Both
Rs and
Rct gradually increased after 60 pattern repetitions. This is due to the battery charge-discharge leading to the increase in IR, the subsequent growth of the SEI layer, consumption of the electrolyte, and the damage on the SEI layer [
11,
12].
3.2.4. Battery Performance Test: DCIR of the “Stop-Operation” Part
In the case of the conventional DCIR, an hour of rest is required for stabilization after setting the SOC to a specific value, and the IR is estimated based on the voltage difference after applying the DC-pulse current. Here, however, the IR is estimated using the “stop” part during the operation of ESS or EV without the time of stabilization. Notably, while repeating each of the V0G, V1G, and V2G patterns, the IR is analyzed at the “stop-operation” part to diagnose battery deterioration. As can be seen in
Figure 8a for V0G pattern, there are 11 “stop-operation” parts with 5 min rest. At the four marked parts shown in
Figure 8a, the IR was compared with the conventional DCIR. The IR can be measured using the voltage of the part to which the current has been applied for
n sec after the rest. In this analysis, the IR was measured using the voltage after 10 sec at the “stop-operation” part.
The SOC at each part was as follows: (1) Part (a): SOC 100%, (2) Part (b): SOC 90%, (3) Part (c): SOC 25%, (4) Part (d): SOC 50%. The SOC at the same part may vary according to the state of battery deterioration. In addition, for every 30 pattern repetitions, the following IRs were measured at SOC 50%: IR of DCIR (
R1) immediately after the application of current, (2) IR of DCIR (
R1 +
R2) after 10 sec of current application, (3) IR of AC-impedance (EIS). These IRs were then comparatively analyzed. Here, the IR of AC-impedance includes both the ohmic resistance
Rs and the charge transfer resistance
Rct.
Figure 8c shows the result of comparing the DCIR at Part (a) and Part (b) and the DCIR at 50% SOC. The DCIR at Part (a) showed a mean difference of 0.218 mΩ from the DCIR at Part (b). For Part (a), the charge at low C-rate allows an adequate level of charge up to 100% SOC. For Part (b), on the other hand, the charge at high C-rate (1.0 or 0.5 C) leads to the charge up to approximately 90% SOC. As the IR varies according to the SOC, different IR values were obtained.
Figure 8d shows the result of measuring the DCIR at Part (c) and Part (d). Similar to
Figure 8c, the IR varies according to the SOC [
8,
9].
In addition, the changes in IR appear highly similar between EIS and DCIR_1s. This is because, as previously said, the IR of DCIR after 1 sec current application includes both Rs and Rct of AC-impedance.
Thus, the analysis of correlation between DCIR and AC-impedance seems to require more precise analysis at interval of 1 ms or 10 ms. Notably, the result in
Figure 8c is considerably important in analyzing the battery deterioration for the running of the EV or ESS. In
Figure 8c, Part (a) or Part (b) are the parts where the system such as EV or ESS operates following the stop so that the IR can be measured from the perspective of actual use and the battery deterioration can be analyzed.
Figure 9 shows the result of comparing the DCIR at the “stop-operation” part according to the repetitions of V1G and V2G patterns. The results for V1G pattern are shown in
Figure 9a,c,e, and the results for V2G pattern are shown in
Figure 9b,d,f. The results were obtained after 330 repetitions of the V1G and V2G patterns. Similar to the DCIR at the “stop-operation” part for the V0G pattern, the DCIR decreased for up to 60 pattern repetitions, followed by an increase based on battery deterioration as the number of repetitions increased. While the resistance varied according to the SOC, the trend was identical across all patterns. Notably, a similar trend of IR based on pattern repetitions could be observed across all of the followings; Part (a), Part (b), Part (c), Part (d), DCIR_1s, DCIR_10s, and AC-impedance, and through this, the changes in IR may be determined, and the state of battery deterioration compared to the initial state may be predicted. Based on the findings so far, it is likely that the DCIR can be measured at the “stop-operation” part for VxG patterns as well as any other operation conditions. Thus, the DCIR method is presumed to be applicable to various ESS or EV operation conditions.