The benefits of the pulsed current technique used to charge/discharge Li-ion batteries have been demonstrated by some researchers. However, the pulsed current charging/discharging strategy does not always have positive impacts on the lifetime and other performances of batteries [
54]. Thus, the pulsed current charging/discharging strategies need to be investigated with different pulsed current mode [
55]. There are four aspects to evaluate the influence of pulsed current on the lifetime of battery cells: (1) Cycle life, (2) capacity loss, (3) impedance of the cell, and (4) maximum rising temperature. For the first point, a reference of the battery capacity
is predefined. Then, battery cells are charged/discharged by a certain pulsed current profile until the capacity reaches
. Finally, the number obtained for life cycles at
is recorded and commonly compared to the number of the cycle life of the conventional CC-CV mode. For the second method, the cycle life test is also performed. The difference for the first method is that the cells are charged/discharged with the same number of cycles under different current modes, then the capacity of the cells is measured and compared. The impedance measurement is also a popular method to evaluate the health and predict the cycle life for battery cells. The impedance of the battery cell is measured by Electrochemical Impedance Spectroscopy (EIS). This method is usually combined with a cycle-life test and is performed after certain cycles, which can represent the resistance evolution over time. The impedance of the battery will increase, while the battery capacity fades. The cycle life of Li-ion batteries depends on the electrode materials and the interfaces among the anode, cathode, and electrolyte. However, the impedance obtained by EIS is mainly related to the surface film and interfacial charge-transfer resistance. Therefore, the impedance of the cell cannot directly reflect the capacity fade. The maximum temperature rising or the peak temperature is regarded as an auxiliary evaluation criterion as a higher temperature rising can result in a negative effect on the battery health during both charging and discharging processes. Thus, the cycle life and the capacity fade are the intuitive ways to evaluate the impact of different pulsed current modes on the battery lifetime. Furthermore, the charging/discharging capacity and the charging speed are also objectives to be investigated. The reason why these two battery performances are analyzed is that the impact of some pulsed current parameters on them is opposite. For example, a higher magnitude of the PPC mode can greatly improve the charging speed, while the charging capacity decreases.
The state of the art of the impact of pulsed current techniques on battery cells is introduced in this section. The first part and the second part are the corresponding impacts of different PPC strategies and different NPC strategies on the lifetime, charging/discharging capacity, and charging speed of Li-ion batteries. The temperature was also investigated in some research. The relevant references are listed in Tables to summarize all research results for readers to review.
3.1. Impact of PPC Strategies
To investigate the impact of pulsed current parameters on the cycle life of Li-ion batteries, JM Amanor-Boadu used Taguchi orthogonal to evaluate several possible impact factors [
16]. The impact factors chosen were frequency (0.1 kHz, 1 kHz, 6 kHz, 12 kHz (
), 50 kHz, and 100 kHz), duty cycle (20%, 50%, and 80%), and ambient temperature (0
C, 23
C, and 45
C), where
is the frequency at the minimum impedance point of the battery cell. It was observed that the cycle life at higher frequencies (
, 50 kHz, and 100 kHz) was higher than the cycle life obtained at lower frequencies (0.1 kHz, 1 kHz, and 6 kHz), and the best result was obtained at
. The impedance parameters were obtained by EIS. As the number of cycles increased, the impedance increased linearly. At a low ambient temperature, a smaller duty cycle led to a smaller cell impedance. Conversely, at a high ambient temperature, a larger duty cycle could obtain a smaller cell impedance. However, in general, the impedance value of the battery operated in the PPC strategy was slightly higher than the impedance value obtained by the traditional CC-CV strategy. The optimal set of parameters for the PPC strategy was the pulsed current with
frequency (12 kHz) and 50% duty cycle at 23
C, which could improve the cycle life by 100 cycles compared to the CC-CV strategy. In another publication by JM Amanor-Boadu [
56], the frequency and duty cycle were the two factors with the largest impact on the performance of the battery. The charging speed at
frequency and 50% duty cycle could be improved by 48% compared to the CC-CV charging strategy. The battery energy efficiency and battery charge efficiency were improved by 12% and 2%, respectively.
The impact of the high frequency on the capacity fade of Li-ion batteries was studied in [
57]. The frequencies chosen were 1 Hz, 10 Hz, 0.1 kHz, 1 kHz, 10 kHz, and 100 kHz. To obtain calendar degradation, float-charging tests were performed and the results were regarded as a reference point to compare with the capacity fade by the pulsed current. After a 147-day cycle life test, the capacity fade ratios at 1 Hz and 10 Hz were about 13% and 15%, respectively, while the capacity fade ratios at high frequency were much lower. The capacity fade ratio at 0.1 kHz, 1 kHz, and 100 kHz was approximately 8%, and at 10 kHz was only 6%, which can be considered as calendar degradation. The temperature rising was measured for different frequencies at a certain voltage level. It was observed that the maximum temperature rise was around 1
C. With the frequency increased, the rising of the battery temperature had a decreasing trend. Therefore, high-frequency pulses did not cause a significant increase in battery temperature.
The frequency and the duty cycle were the two variables used to investigate the impact of the pulsed current strategy on the cycle life for lithium-metal batteries in [
58]. The frequencies selected were 0.17 Hz, 0.03 Hz, and 0.017 Hz. The duty cycles chosen were 66.7%, 50%, 33.3%, 25%, 16.6%, and 9.1%. As the duty cycle decreased, the cycle life was prolonged and reached the maximum cycle number at a 16.6% duty cycle. At a duty cycle of 9.1%, the cycle life had a decrease. Higher frequencies have a more positive effect on battery life. When the duty cycle was 16.6%, the cycle life was improved by about 55%, 70%, and 130% at the corresponding frequency of 0.017 Hz, 0.03 Hz, and 0.17 Hz compared to the cycle life obtained by CC-CV strategy.
The PC-CV strategy with 0.02 Hz frequency and 50% duty cycle were compared with the CC-CV strategy at different current levels (1 C, 2 C, and 3 C). After 100 cycles, the cells tested with the lower current rate had a higher capacity retention rate for both the pulsed current mode and continuous current mode. When the pulsed current mode and continuous current mode were compared at the same current level, the capacity retention rates of the pulsed current mode were improved slightly (0.26%) compared to the continuous current mode [
45].
The CC-CV charging strategy was compared to three pulse charging strategies, which are CC-PC charging strategy, PCCC with 1 Hz frequency charging strategy, and PCCC with a 25 Hz frequency charging strategy [
37]. The mean current of the charging strategies was the same for both continuous current mode and pulsed current mode. The cycle life of CC-PC and PCCC with 25 Hz frequency was similar to the one obtained for the CC-CV method, while 1-Hz PCCC had a faster capacity fade than the CC-CV method. The capacity utilization and charging speed were also investigated in this study. The capacity utilization of the three pulse strategies was lower than that of the conventional one. This results obtained were because the maximum charging voltage was set lower than that of CC-CV in case of the high overvoltage during the charging process, especially with the high current pulses. This means that the process of pulse charging ended at a lower SOC that led to a low charging capacity. For the charging speed, the CC-PC strategy had a longer charging time because there was relaxation time after the CC stage, while there was no relaxation time for CC-CV strategy. The charging speed of PCCC strategies was not impacted because the same mean charging current was used for the CC-CV charging mode.
The PPC charging mode with four different frequencies (1 Hz, 100 Hz,
, and 10 kHz) at 50% duty cycle was investigated and compared with the CC-CV charging mode, where
was 998 Hz, 996 Hz, and 1238 Hz for three different batteries obtained by EIS [
21]. The pulsed current at
presented the best performance for all batteries. The charging speed, discharging capacity, and efficiency at
were average improved by 16.2%, 2.1%, and 1.6%, respectively. Moreover, the average rising temperature of the three batteries, which were tested using pulsed current was 2.8
C, which is lower than the average temperature rising (4.4
C) obtained by testing the battery using continuous current. Thus, the high frequency did not lead to a high temperature rising and had a positive trend compared to the traditional charging strategy.
The current amplitude (0.5 C, 1 C, and 2 C), pulse time (1.2 s, 1.5 s, and 2 s), and relaxation time (0.3 s, 0.5 s, 0.7 s, and 1 s) were selected as variables to investigated the impact of the PPC strategy on the performance of batteries [
59,
60]. With the increase of the pulse amplitude from 0.5 C to 1 C, the charging time decreased by 82.8%, while the charging capacity decreased significantly (32.9%), and the maximum temperature increased by 15.3
C. The increase in the pulse time from 1.2 s to 2 s could also decrease the charging time by 14.1%. The changes in relaxation time had no significant impact on the charging capacity and temperature rising. However, the increase in the relaxation time resulted in a longer charging time. The effect of relaxation time on the recovery capacity for pouch Lithium-sulfur battery cells was studied in [
61]. During the discharging process, a longer rest time led to a higher discharging capacity. The effect of the capacity recovery rate was more significant with a higher discharge current and lower ambient temperature. The relaxation period could recover the battery capacity up to 20%.
The impact of PPC strategies on the lifetime for Li-ion batteries is summarized in
Table 1. Reference [
58] is not presented in
Table 1 because in that work, lithium-mental batteries were used. The impact of PPC strategy on the charging speed and charging/discharging capacity for Li-ion batteries are listed in
Table 2.
Table 3 is the impact of various frequencies on temperature. Further analysis and discussion are conducted in
Section 4.
3.2. Impact of NPC Strategies
The effect of the NPC strategy on the life performance of Li-ion batteries was studied and compared with continuous current charging in [
48]. The author claimed that the negative pulse and the rest time could improve the active material utilization, which would result in a higher discharging capacity and longer lifetime.
was defined to be used to compare the cycle life of the cells operated in the NPC strategy and continuous current strategy. The cell charged with the NPC profile experienced 1600 cycles before the capacity decreased to
, while the cell charged with continuous current experienced only 700 cycles before the same capacity was reached.
In [
52], CC-CVNP protocols with different amplitudes and a different number of negative pulses were built to investigate their impacts on the lifetime. The results showed that CC-CVNP with lower amplitude and fewer number of the negative pulse could reduce the capacity loss. The internal temperature of the cells was stable, and the surface temperature was not affected significantly since the temperature rising was around 1
C. Impedance measurements were performed and the obtained results demonstrated that the CC-CVNP with the lowest amplitude and the fewest numbers of negative pulses could reduce the impedance of the battery cell. Compared with other charging algorithms provided in this study, the CC-CVNP with the lowest amplitude and the fewest numbers of negative pulses could effectively slow down the aging process.
The lifetime of the MCC-CVNP strategy and of the CC-CVNP with different numbers of negative pulses strategies were evaluated using a certain procedure in [
53]. Compared to the CC-CV strategy, the capacity retention improvement of the MCC-CVNP was 8%, but it was lower than that of the MCC-CV strategy and the CC-CVNP strategy, which could improve the capacity retention by 13% and 11%, respectively.
The negative pulse magnitude
(0.5 C, 1 C, and 2 C) and negative pulse time
(0.2 s, 0.3 s, and 0.5 s) were considered as the impact factor to evaluate the effect of the NPC strategy on the performances of Li-ion batteries in [
59]. With the increase of the negative pulse time, the charging capacity increased by about 3.3%, while the battery charging time increased by 36.9%. The rising temperature was hardly affected. Similarly, the increase of the negative pulse amplitude did not have a significant positive effect on the charging capacity, but the charging time and battery temperature increased by 16.4% and 14.2%, respectively. Thus, the increase in the level of amplitude and time for negative pulse had a negative impact on the performances of batteries.
To investigate the effect of ACP charging strategy on performances of Li-ion batteries, the mean current (5 C and 20 C), the temperature (–10
C and 25
C), the frequency (200 mHz–100 Hz), and duty cycle of the negative pulse (1%–10%) were considered as factors in [
50]. It was observed that the pulse discharging had no positive impact on battery performance. Moreover, compared to the CC-CV method when discharging the battery cells using a certain mean current, the pulse strategy had a negative effect on the battery’s performance.
In [
62], the charging time at a different SOC range was investigated considering the NPC strategy. For the SOC range 0%–40%, the charging time was shortened by 50% compared to the continuous current charging, while for the SOC range 0%–60%, the charging time was reduced by 43% compared to the continuous current charging. After 60 cycles, the capacity fade reduced by 23% compared to the CC-CV strategy. Although the charging time of the NPC at full SOC range was slightly longer than the CC-CV strategy, the charging time of the NPC was less than the CC-CV after 40 cycles. Moreover, the capacity fade could be decreased by 23% compared to the CC-CV method after 60 cycles.
The impact of NPC strategy on the lifetime of Li-ion batteries is summarized in
Table 4.
Table 5 presents the impact of NPC strategy on temperature behavior. Further analysis and discussion are conducted in
Section 4.